The Principles of Aquarium Keeping

The construction of aquaria is an art in itself and, like the details of the various individual species of fish, will not be discussed in this volume. For most amateurs, it is cheaper in the long run to purchase suitable tanks than to make them, and generally it is more satisfactory. Aquaria for decorative purposes should be made from clear glass, without the irregularities and flaws frequent in ordinary window glass, and the glass should be thicker than window glass in any but the very smallest tanks. However, for aquaria less than 12 inches deep and 15 gallons in capacity, it is not necessary to use plate glass for the sides, back, and front. These can be of 28- or 32- ounce glass, with a 1/4-inch cast or plate glass bottom, or a bottom of slate. Larger aquaria with a depth up to 18 inches and a capacity of about 40 gallons can be made of 1/4-inch plate or cast glass throughout, as long as the bottom is in two sections in tanks more than 30 inches long. Above 40 gallons and 18 inches in depth, thicker plate glass must be used, 3/8-inch plate with a 1/2-inch plate bottom is sufficient up to 100 gallons and 24 inches in depth.

Shape and Size of Tanks

Aquaria should be as shallow as possible, since the surface of water exposed to the air is the most important factor in determining the number of fishes they can safely hold. However, a very shallow tankis an eyesore, and a compromise is always made between biological and artistic requirements. Many prefer a "double cube" type of construction, with the tank twice as long as it is wide and high, i.e., 24 X 12 X 12 inches. This tank is still rather shallow for the full growth of plants and looks better if the height is somewhat increased,
so that a common variation is 24 X 12 X 14 or even 24 X 12 X 16 inches.

Such tanks can hold no more fishes than the first-mentioned, they merely look nicer. The market offers anything from about 10 X 6 X 8 inches to 36 X 18X18 inches, or even larger. This is a range of 2 to 50 gallons and covers all needs of the average aquarist. A recent modification of the rectangular tank is the bow-fronted model, with a "bay window." It is not to everyone's taste and is costly.

Except for special purposes, very small tanks are a mistake, as they give little scope for decorative effects, are difficult to keep at an even temperature unless in a fish house, and hold few fishes per tank (although they allow the maximum per gallon, because of their increased total surface area), About the minimum useful size for an ordinary community tank is 6 gallons (18X8X10 inches). To compute gallon capacity, multiply the length, breadth, and height in inches and divide by 231, thus:
(18 X 8 X 1O)/231 = 1440/231 = 6.2 American gallons

This refers to the actual volume of the tank, and, if allowance is made for a 1-inch air space on top and 1 1/2 inches of sand at the bottom, the actual water volume is only about three-fourths of the calculated volume in this instance.

Fish Capacity of  Tanks

In Chapter I mention was made of the "balanced" aquarium and of the reciprocal actions of plants and animals. Until recently it "was believed, however, that the exchange of carbon dioxide and oxygen between fish and plants, taking place directly through the water, was more important than is really the case. In a crowded tank, with plants in a good light, this interaction matters considerably, and the same tank is often in a poor state at night. With a tank not unduly crowded, either with fish or with plants, the exchange of gases between the air
and water is more important than any other factor. This is why the surface area of the tank counts for so much and why, in practically all circumstances, the influence of plants may be ignored when fish capacity is considered from a respiratory point of view.

In addition to surface area, surface movement and the circulation of water within a tank are important, and that is why an aerated tank can hold more fishes than a still tank. As we shall see in Chapter VII, the fact that the  movements of the water in aeration are produced by air bubbles is usually of little consequence, and it is an example of getting the right result for the wrong reason. The old idea behind aeration was, of course, to increase the contact of air and water by sending fine bubbles coursing up through the tank. Unless a very brisk spray of very fine bubbles is used, however, the surface of water ex-
posed to air in the bubbles is small and unimportant, and the movement caused at the tank surface is what matters.

Thus, we compute fish capacity from surface area, and we may modify this by including the effects of temperature, water movement, and various other factors. The warmer the water, the lower the solubility of oxygen, and thus the lower the fish capacity. The following estimates assume an average temperature of about 75°F. for tropicals and about 60°F. for cold-water fishes. They assume no aeration, and they also assume that young fishes and small fishes, even though adult, use more oxygen per unit of body weight than do larger fish. This is in line with such experimental details as are available, though they are scanty, and with general experience. They are not based on the "gallon" or "inch of fish" rules, which give various estimates of the number of inches of fish per gallon which may be placed in tanks, because this type of computation is clearly fallacious and is not in line with either the practice or the experience of  observant aquarists.

The general basis of the estimates is as follows. It is assumed that, age and activity apart, the same volume or weight of fish uses about the same amount of oxygen per minute, whether it comprises a hundred small fishes or one big fish. However, small fishes and young fishes are usually more active and young fishes are still rapidly growing. They therefore consume more oxygen weight for weight, and allowance is made for this to the extent that 1 ounce of 1/2-inch fishes is allowed five times the oxygen consumption of 1 ounce of 21/2 –inch fishes, and each ounce of 6-inch fishes is allowed rather less oxygen than that of 2 1/2-inchers. The base line is the 2 1/2 -inch tropical fish, which is allowed 20 square inches of surface area—a generous allowance— and all estimates should be divided by 3 for cold-water fishes and by 6 for fancy goldfish varieties. The 20-square-inch allowance is intended to permit further growth and good health. The fishes would not be expected to show distress if their numbers were doubled, but they would not flourish so well. The estimates are clearly approximate and could be misleading in special cases, but from experience it is felt that they are a much better guide than other current recommendations. It will be apparent to the thinking reader that fat fishes of the same length are likely to use more oxygen than slimmer fishes, but this fact is fortunately minimized by the greater activity and therefore greater oxygen need of the slimmer, minnow-like types. One major alteration in the rule applies to the Anabantids, which, when the labyrinth is developed, require only half the surface area per fish otherwise recommended. (The labyrinth is present in all but the
very young fry.)

Body length offishes in inches*	No. of fishes per square foot(144 square inches) of surf ace area	Square inches per fish
1/2180	0.8
3/4	100	1.4
1	55	2.6
11/2	20	6.6
2	12	12.5
21/27	20
3	5	30
4	2	6-5
5	1	120
6	1	180

* The body length excludes the tail fin.

Example 1. In a 15-gallon tank, with a surface area of 288 square inches, the following fishes are to be housed:

8 characins, each about 11/2 inches long
12 characins, each about 2 inches long
4 barbs, each about 2 inches long
6 barbs, each about 2 1/2 inches long
6 gouramis, each about 2 1/2 inches long
2 cichlids, each about 3 inches long

Are they likely to be overcrowded, or can even more be dded?

We have:
8 fishes at 1.5 inches need 6.6 X 8 =52 square inches
16 fishes at 2 inches need 12.5 X 16 = 200 square inches
6 fishes at 2.5 inches need 20 X 6 =120 square inches
6 gouramis at 2.5 inches need 10X6= 60 square inches
2 fishes at 3 inches need 30 X 2            =60 square inches
Total = 492 square inches

Conclusion: The fishes are crowded and cannot be expected to be at their best. Certainly no more should be added, and it would be best to omit a few.

Example 2. What size tank should be used to house 100 fry 1/2-inch in length,  with room to grow them to 3/4inch before moving them again? It is the final size of fry that matters; hence the tank must have a surface area of 100 X 1-4 square inches, or approximately 1 square foot. If they were to reach 1 inch, then an area of 260 square inches or approximately 2 square feet would be needed, i.e., a 15-gallon tank.

Modifying Factors

The addition of plants to the aquarium does not much affect the calculations above. If the fishes are not unduly crowded, then additional daylight help from plants is unnecessary, but no disadvantage, and their nighttime absorption of oxygen will not much matter, particularly as the fishes are inactive and so consume less oxygen and produce less carbon dioxide themelves. If the tank is crowded, the daytime activity of the plants makes all the difference, but at night, despite the fishes' quietude, the added burden of plant respiration is likely to make conditions very hard. This may be observed in many a crowded and heavily planted tank; while daylight lasts or artificial illumination is supplied, the fishes are quite comfortable, but at night, and very soon after they have been plunged in darkness, they are gasping at the surface. They may and usually do survive this treatment, but the nighttime stress should be relieved by aeration or, better still, by lowering the fish population of the tank. There is good reason to believe that the accumulation of excess of carbon dioxide is more important in most circumstances than is lack of oxygen and that the dark, overcrowded tank is suffering more because the plants are failing to remove carbon dioxide than because they are failing to produce oxygen. Aeration helps because some of the carbon dioxide is "blown off" by the water movement and bubble contacts. Throughout this book, there are many places where a statement about oxygen deficiency or competition for oxygen should really have a reference to carbon dioxide excess added or substituted, although the practical result is the same.

From the above, it will be clear that the best solution for increasing fish capacity is water movement, usually supplied by aeration. This can double the safety margin, so that the fish population may be doubled in a well-aerated tank, but only if the tank is also well- planted and clean or if the water is partly changed frequently to eliminate waste material. Water movement tends to  stir up waste material and aid its more rapid solution in the water, and adequate provision for purification is needed. As will be seen in Chapter VII, a filter does not necessarily fulfil these requirements, as it may act as a very efficient dissolving plant for the waste products and help to poison the water even though it keeps it clear. The problem requires further experimental work before we really know what to recommend with confidence other than water changes and adequate plant life.
One of the factors that definitely tend to reduce fish capacity is the presence of waste materials, especially unconsumed food or dead plant material, less so actual excretory products of the fishes. All these substances use oxygen and produce carbon dioxide in continuing decomposition and thus reduce the safety margin for the fishes. Another factor is the presence of invertebrate animal life other than air breathers. Colonial sponges or polyps or an excess of unconsumed living worms or crustaceans may make conditions dangerous for fishes.

A final important factor is temperature. Oxygen dissolves more readily in cooler water, so that at 50°F. a gallon of water can hold about 1.8 cubic inches of oxygen (or the oxygen from about 9 cubic inches of air) whereas at 80°F. it holds only 1.3 cubic inches. This, by the way, is the maximum oxygen capacity, assuming pure water without the inevitable presence of carbon dioxide and other gases which replace some of the oxygen in ordinary circumstances. Thus, in the average tank, a content of around 0.7 cubic inch per gallon is quite good and adequate for the fishes; some can take down to about 0.3 cubic inch without severe distress. Meanwhile, the fishes are becoming considerably more active and their rate of consumption of oxygen rises, both because of this and for more fundamental chemical reasons, with the net result that the fish capacity of the tank falls quite rapidly with increased temperature. In tropical tanks, which do not usually experience a wide temperature variation, this is not likely to matter much, but in a cold-water tank which undergoes a considerable rise, as on a hotter day than usual, serious results may occur. As far as is known, fishes resemble other cold-blooded animals in that their metabolic rate (the rate, among other things, of oxygen consumption) is about doubled for every 18°F. rise, so that a rise from 60°F. to 78°F. can tax a crowded tank so much as to cause deaths.

The question of balance in a fish tank is therefore a relative affair and not as pictured by the average aquarist. The greatest "adjusters" are the fishes themselves, which can vary their respiratory rate about ten-fold, and thus cope with a wide range of oxygen availability or carbon dioxide excess. This does not mean, however, that they can live in a tenth of the normal oxygen concentration, as the rate at which the gills can take it from the water falls sharply with a decrease in concentration, so that the average fish is in distress when a 50% fall occurs, as shown above. That is why some fishes, particularly those used to running water or the open sea, cannot stand ordinary aquarium life, as the oxygen content even at best is below their tolerance.

Water Quality

Water from the faucet may be perfectly satisfactory for the aquarium immediately as it is drawn, but often it is not. It is likely to contain free chlorine, especially in city areas, and it is possibly under sufficient pressure to contain an excess of other dissolved gases, although a poisonous excess of oxygen, sometimes claimed to be a danger, seems unlikely. It may be of the wrong salinity or hardness, and it may be too acid or too alkaline. However, in most districts tap water is perfectly satisfactory after standing for a day or two, a process sometimes called "conditioning." This allows for gas exchange between the water and the air, which may be hastened by aeration or by boiling
and cooling again, but dangerous pH changes may follow the latter.

Chlorinated water may be rendered immediately safe by the addition of 1/2grain of sodium thiosulphate (the "hypo" of photographers) per gallon of water, as far as its chlorine content is concerned, but this does not guarantee its immediate suitability in other ways. Distilled water or even filtered melted snow may be used; if so, salts should be added, as such water is too "pure" and will not support life adequately. Suitable salts are:
3 teaspoons (3/4 ounce) of common salt
1 teaspoon (1/4 ounce) of magnesium sulphate
1 teaspoon (1/4 ounce) of potassium sulphate 
per 10 gallons of distilled or snow water.

Rain water may need similar treatment, but it is dangerous to use in towns, for on the way down it collects a lot of smuts and harmful chemicals from the  air and may be very toxic. Snow is less liable to be so polluted, even in towns.

Hard water contains dissolved salts absent, or relatively so, from soft water. Very soft water should have salts added as for distilled water, whereas normally hard water doesn't need them. For orientation purposes, here are typical analyses of sea water, hard water, and soft water, in percentages of various important salts:

Aquarium water can with advantage contain rather more salts (but not more calcium) than typical hard water ( except for special purposes, such as the breeding of neon tetras), and up to 0.1% of total solids is quite in order. Most fish can take much more.

Hard water is usually alkaline; soft water is usually neutral to acid in reaction. Acidity and alkalinity are measured on a scale which goes from 0 to 14, called the pH scale. Neutral water has a pH of 7, acid water has a pH of less than 7. Strong acids are down in the 1 to 2 region, strong alkalis up in the 12 to 13 region. A weak acid like carbonic acid (dissolved carbon dioxide) even at full strength has a pH of only about 4, a weak alkali like sodium bicarbonate a pH of about 9, and this is approximately the range seen in natural waters. Few fishes can stand this entire range, but most can be happy anywhere between 6 and 8, and many do not show distress when well beyond these limits. Most aquarium plants flourish best at a slightly alkaline pH. There was a period of emphasis on pH in aquaria, when differences of 0.1 or 0.2 were thought to matter, but a common-sense view of the problem is now more usual, and few worry about the pH value of ordinary successfully maintained tanks. During breeding, pH may matter more, as the germ cells or fertilized eggs and fry do not necessarily tolerate as wide a range as the adults, but very little is known on this question. The pH of a tank is not usually constant and tends to rise during the day, as carbonic acid gas is removed by the plants, and to fall at night, when both plants and animals produce it. The extent to which this affects pH readings depends on the degree of aeration, the atmosphere in the room, and the fish and plant density. In a crowded tank in a room with rather foul air, such  as an exhibition tank at a crowded indoor show, the pH may fall alarmingly in a few hours to values of 5.5 or lower.

To measure pH, chemicals called indicators may be used. These are added dropwise to a small sample of aquarium water and change color according to the pH. Thus, the commonly used bromothymol blue is yellow when at a mildly acid pH, green when neutral, and blue when

alkaline, covering a useful range of about 6.0 to 7.6. For sufficiently accurate work, color charts are supplied in steps of 0.2 or 0.3. A newer departure is the comparator paper, such as Johnson's Comparator Test Papers, which are dipped straight into the tank and compared with a chart afterwards.

To adjust pH, when really necessary, the acid and alkaline sodium phosphates are best, but sodium bicarbonate may be used on the alkaline side instead. Such adjustments must be made with care, in the following way.

Make a solution of sodium monohydrogen phosphate (Na2HPO4, the alkaline  phosphate) at a concentration of 1 in 100 (1 gram per 100 milliliters or 1 1/2 ounces per gallon, approximately), and a similar solution of the dihydrogen phosphate (NaH2PO4, the acid phosphate). When mixed in equal anhydrous quantities, these two solutions make up a buffer solution with a pH of 6.9 approximately, and a capacity for holding its pH against the addition of other salts. Such a buffer solution may be used for holding an aquarium pH steady, but its capacity is not unlimited and the salts are gradually broken down. If so used, not more than about 1 part in 20 of the mixture should be added to aquarium water, which should first be brought to a neutral pH. To bring a tank to neutral pH or any other desired pH, a sample of the water, a definite quantity such as 1 pint, should be withdrawn. If the pH is too low, i.e., if the water is too acid, the alkaline phosphate solution described above is added in small amounts with frequent testing until the desired effect is obtained. Then, by simple proportion, the amount to be added to the whole tank is determined. This addition should be made with constant stirring. If the pH is too high, the acid phosphate solution is used in the same manner. When making such adjustments, do not forget to compute as accurately as possible the true water volume, making allowance for space occupied by sand, rocks, etc., and for the top level in the tank. Lastly, do not make drastic alterations in pH all at once. Charges of more than about 0.5 per day are to be avoided, and for complete safety it is probably best to make adjustments in steps of not more than 0.2 to 0.3 at a time.  We don't actually know for certain what the limits are, as in so many of our aquatic problems, so we must play for safety.

New and Old Water

The earlier aquarists were great conservationists with their aquarium water. When they siphoned off detritus and mulm, they usually filtered it and returned as much as possible to the tank. As a result, the water gradually acquired a rich, wine color and a limpid clarity. The latter is very attractive, and it is surprising how yellow or even reddish the water can become without causing comment, unless a sample is compared with fresh, colorless water, when the difference is very obvious.

This "aged" water was supposed to have become more suited to the fishes than fresher (although not necessarily completely new) water. The thesis was, and still is for that matter, that fishes make water in which they live more suitable for themselves and even poisonous to  other species not present in the tank. There seems to be little foundation for this belief, and it is certain that the same water, kept long enough, becomes poisonous to all fishes, even in the presence of growing plants. Toxicity to other species may well be the result of their inability to take a sudden exposure to water to which the permanent inhabitants have become gradually accustomed, as it has become steadily charged with waste products. However, the question is not settled and would repay more study. It has even been suggested that some "animal protein" factor is involved, but this is again very conjectural. Both expectation and experience now concur in advising a steady change-over of tank water. This may not need to be very much; usually 5 to 10% per week or fortnight is ample, but in crowded, badly illuminated tanks a change of 20 to 30% per week may be a decided advantage. Even in winter, when the gas content of fresh tap water is highest, the danger of such a volume of replacement is negligible, but, if preferred, the water may be left to stand for a day or two before use. Of course, the new water must be at the right temperature; never rely on the heater to take care of sudden additions of cold water to a warm tank. Even if the fish escape immediate chilling in the cold stream, the over-all temperature after mixing may have fallen dangerously.

The qualities implied by the term "old water" are often therefore not really age but suitability. It is water which does not contain noxious gases or solids, is at the right pH, has the right salinity and hardness, and the right content of algae, infusoria, and perhaps bacteriophages. And the word "right" differs according to whether the water is intended for a community tank, for spawning a particular species, or for raising fry.

Cloudy Water

Water may become cloudy for a number of reasons. Green cloudiness is due to the presence of green algae, usually brought on by too much light in the presence of organic materials in solution. It is fundamentally healthy water, but it spoils the appearance of a tank and is usually not desired, except for raising fry. If it becomes very thick, there is danger that the rapidly growing algae will suddenly die and foul the water. The first sign of this is a slight yellowing of the water, which then rapidly turns fetid and is very dangerous to the fishes. It should be entirely changed without delay, using fresh tap water at the right temperature rather than taking the risk of leaving the fishes as they are. A store of spare "aged" water is commonly kept by experienced aquarists for just such an emergency.

Cleanliness, moderate light, no crowding of tanks, and the presence of plenty of higher plants, especially floating plants, help to prevent the appearance of green cloudiness. Tanks relying wholly or predominantly on electric illumination practically never develop green water, mainly because they do not receive excessive illumination. Acidity of the water makes for cleanliness, but not for good plant growth, so that both the algae and the higher plants are discouraged together. It is often said that a partial or nearly complete change of water only serves to stimulate further algal growth; however, this does not always occur, and green water may be cured immediately by siphoning it off—plus any debris in the tank—and replacing it with fresh, conditioned water.

Gray cloudiness may be due to infusoria, bacteria, fungi, or just dirt. It often follows the setting up of a tank, sometimes because the sand was not washed thoroughly, but often because there is a burst of animal life which starts ahead of the algae. This cloudiness, whether dirt or infusoria, usually disappears in a few days and is not dangerous. Bacterial or fungal cloudiness is a bad sign, usually seen in established tanks which are "going bad." It may be caused by excessive decaying matter, usually dry food but sometimes a dead fish or mollusk, by insufficient light, by overcrowding, or by the use of unsuitable gravel or rock, which may provide niches and crannies where excreta and food are caught and decay. Although attention must be paid to the basic causes, such as overcrowding, gray cloudiness may often be temporarily corrected by brisk aeration or by the addition of disinfectants and antibiotics. Another effective method with both gray and green cloudiness is to place one or two large fresh-water mussels in the tank for a few days. These will clear water when a filter will not.

Removal of Mulm

In the course of replacing some of the water at intervals of one or two weeks as recommended, it is easy to siphon off much of the mulm from the bottom of the tank. A convenient way to do this is to take a glass tube of 1/4 to 1/2 inch in diameter, slightly shorter than the aquarium depth if it is left straight, or a little longer if the top is given an angle bend, to prevent the rubber tubing which is attached to the glass tube from kinking. The business end of the glass tube should be softened by heat and then gently pressed onto a flat surface, holding the tube upright, so that the entrance is slightly constricted, to prevent
pebbles, snails, and so forth, from getting into the tube and clogging it up. Any that do enter will be small enough to pass readily through the siphon. The rubber tubing should reach for 2 or 3 feet outside the tank below the water level, so as to give a convenient head of pressure.

The siphon is started by sucking the end of the rubber tubing with the glass tube immersed, or, if preferred, by filling it with water by dipping in the tank. With a little practice, it is possible to start the siphon without getting a mouth full of water. The rate of flow is controlled by pinching the rubber tubing, and the siphon is then run gently over the aquarium bottom like a vacuum cleaner. The end of the glass tube may also be poked well into the sand, sucking up sample "bores" to see if all is well and to see that underneath the surface the sand is not becoming foul and gray with bacteria. If it is, it is wise to go on siphoning the sand away until the foul patch is removed and to replace it with fresh sand afterwards. With experience, it is possible to remove three-quarters of the bottom of a tank, leaving the plants undisturbed, and to replace it by running clean dry sand down a funnel under the surface of the water. If the sand is wet, it sticks and must be washed into the tank, which of course may be done, or it may be put in by hand and the plants afterwards well shaken to remove sand grains.

The foul sand may be washed and dried off and left in the sun, or in a bleaching solution such as is used for washing clothes, and used again. The base of the siphon may be flared out like a small funnel, which, if plunged into the sand, does not lift much or even any of the actual sand but draws out  other matter and helps considerably in preventing the major operation just  described. However, a well set-up tank doesnot often develop serious sand trouble in any case.

There are a number of aquarium gadgets on the market for the mechanical removal of mulm and other materials. The most popular is the filter, dealt with in detail later, but there are also little "vacuum cleaners" available for use with an air pump instead of the single siphon just described. They may be very efficient in clearing the mulm and clarifying the water, but their use does not involve the removal of some of the water, which should be done even though little or no visible mulm remains to be siphoned off.

Removal of Fixed Algae

Green algae will cause not only green water, but filamentous forms will coat the  inside of the glass if there is too much light. These algae may be removed with a scraper, essentially a razor blade on a convenient handle, or, more easily, with steel wool. Wrap a small ball of steel wool around the end of a stick and rub the glass with moderate pressure. It usually takes only a couple of minutes to clean a large tank thoroughly. When removing algae at the base of the tank, be careful not to get sand caught in the steel wool, as it may scratch the glass badly.

Under electric illumination, blue-green algae are more likely to be a nuisance, settling not only on the glass, but also on the plants. These may be scraped from the glass as usual, but to remove them from the plants is almost impossible. Cut down the light and hope that it disappears, or remove all fishes, snails, etc., and treat with one part per million copper sulphate (crystalline) for a few hours, afterwards removing nearly all the water and refilling before introducing fishes again.

Routine Checks

The aquarist gets to know the look of a healthy tank, but even after much experience he may miss the early signs of trouble if he doesn't look carefully for specific symptoms. He must learn to look at the component parts of the set-up and to notice anything that may spell trouble so that corrective action may be taken at an effective stage. Points to remember are:

The smell of a healthy tank is earthy and pleasant. Look for the cause of any departure from this.

The plants should be of good color and not decaying in part or whole. Remove any large decaying leaves, stems or roots and test the pH if decay continues.

The water should of course be clear or mildly green, and its surface should be clean and without a film of dust, oily material or bacteria.

The sand should be clean, loose, and not gray below the surface. If it is disturbed, no bubbles should rise from it.

The fishes should look alert and well, with fins clear and held away from the body. "Clamped" fins or peculiar swimming motions mean impending trouble. The fishes should have no blemishes or strings of excreta hanging from them.

The corners of the tank and hidden crannies should be searched for dead fish or sick fish hiding away. The temperature should be checked as a daily routine in tropical tanks.

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