Aeration, Filtration, and Circulation of the Water

There are probably more misconceptions about aeration than about any other section of aquarium lore; indeed, as we do not fully understand the effects of aeration in relation to the action of plants, etc., this is not surprising. We do know, however, that aeration does not force air (or oxygen) into solution in the water, which is the most popular misconception. Nor, under usual conditions, is much oxygen absorbed from the bubbles themselves; they merely serve to stir the water, and a small propeller would perform the same function. Oddly enough, this method never seems to be employed. It would therefore be superfluous to use oxygen rather than air, and certainly much more expensive, unless such a fine jet at powerful pressure were used that the tank looked like a soda fountain in action.

The accounts in the literature of the rate at which aeration of different types  rings about the solution of oxygen from the air in water that is deficient in oxygen are conflicting. The general conclusion seems to be that it is not as efficient as we have tended to think, some investigators finding that only very brisk bubbling from several aerating stones at once will bring a tank up to 75% saturation within 30 minutes. The normal rate of aeration—a mild trickle from one stone—was found to be ineffective even within several hours. This need not alarm us unduly, because the rate at which carbon dioxide needs to be "blown off" is quite low, and this is probably the most important factor. The fact remains that quite mild aeration about doubles the fish capacity of a tank, and although brisk, boiling aeration might quadruple it or even much more, as far as air content is concerned, other factors would prevent us from trying to crowd our tanks so much. Chief among these would be pollution from excessive fecal and other matter.

Aeration Stones

The usual method of aeration is to force air through a porous "stone" at the bottom of the tank; such a stone may be made of various substances, from fused glass or natural minerals to leather, wooden or felt barriers on a metal head. Carborundum stones give about the finest bubbles but also need the most powerful air pressure, so that only the larger-sized pumps are satisfactory for their use. All stones tend to clog, especially when not used at much pressure, and they should occasionally be removed, thoroughly dried (even baked), and reset. When they are first used or re-used, it is advisable to send a brisk jet of air through them for a few hours, and when this is turned down a fine spray of bubbles is usually the result.

Fine rubber or plastic tubing is used to lead air from the pump to the stone or stones, and various connecting pieces, valves, and clips to control the rate of air flow are on the market. The most satisfactory tubing is one of the finer grades of electricians' "spaghetti," or plastic insulating tubing, which is available very cheaply from radio stores. Even dust filters for inserting into the air line are available, also pressure gauges, but the average aquarist will not find them of much help.

Factors Affecting Efficiency

Experience shows that with reasonably small bubbles, of an average diameter of about 1/25 inch or less, an aerating stone delivering 2 cubic inches of air  per minute is adequate in a 15-gallon tank. This observation is really of great interest, for it demonstrates the fact that adequate aeration does not work by simple bubble-water interchange, for the surface of the bubbles exposed to the water at any one moment is only about 12 square inches, assuming a bubble diameter of 1/25 inch and a period of 4 seconds for a bubble to travel from bottom to top of the tank. As the bubble size decreases (a really first-class stone can give bubbles of less than 1/100 inch in diameter), the surface exposed by the bubbles rapidly increases, for not only is it greater per square inch of air, but also the bubbles take longer to reach the surface.

With 1/100-inch bubbles, which take some 10 to 15 seconds to travel up through the water, the area exposed is 120 to 180 square inches, but even then it is less than the normal surface area of the tank (288 square inches). With such fine bubbles, the water looks quite cloudy over the stone, for at any one moment as many as 1 million are suspended in the water. When the bubble diameter is 1/25 inch, a mere 2,000 odd are seen at once.

With 1/100-inch bubbles, which take some 10 to 15 seconds to  travel up through the water, the area exposed is 120 to 180  square inches, but even then it is less than the normal surface area of the tank (288 square inches). With such fine bubbles, the water looks quite cloudy over the stone, for at any one moment as many as 1 million are suspended in the water. When the bubble diameter is 1/25 inch, a mere 2,000 odd are seen at once. Furthermore, such very fine bubbles do not move water so much and do not disturb the surface of the water, and thus the surface interchange is little affected; therefore the effects of very fine subdivision of the same volume of air may not be as good as rather coarser subdivision. It is the opinion of some experts that a bubble diameter of about 1/3o inch is the best.

Deep tanks gain relatively more from aeration than do shallower tanks, for not only are they in more need of it but, since the bubbles take longer to rise to the surface, they cause more effective stirring of the water and a brisker surface interchange.

Types of Air Pump

The commonest aerator is a pump, operated electrically. Pumps are designed to deliver anything from 10 to 200 cubic inches of air per minute and thus operate from a few to a hundred stones. The electrical consumption of the most powerful is quite small, no more than 30 or 40 watts; the smaller pumps take as little as 3 watts and thus cost next to nothing to run. In fact, if an electric meter is not in first-class order, a 3-watt pump working alone does not move it.

Pumps may be of single or multicylinder type, or without cylinders at all, operated by a vibrating diaphragm. Those without cylinders are the cheapest to run and have been perfected recently by a number of manufacturers to such a  tage that they are often preferred to the older cylinder pumps. They do not require oiling and are more silent,  or should be, but they are less durable and must be placed above the level of water in any tank they supply, or water may run back into the pump. On the other hand, they do not reverse, as do some pumps of conventional design, and so cannot actively suck the water into themselves.

Air may be compressed into drums, and these can last for quite a time. Even a 5-gallon drum pumped up to a pressure of 1 atmosphere (14 pounds to the square inch), half the pressure in a car tire, will deliver a total of about 1,100 cubic inches of air. This is enough to operate an aeration stone for about 10 hours, but the air would not flow at a uniform rate throughout this period. A better arrangement is to have water flowing from one reservoir to another, displacing air at a more uniform pressure throughout the period covered. A more efficient, small electric pump is a much easier solution and doesn't cost very much.

Other Aeration Methods

Various forms of spray or drip have often been used for aerating tank water. A steady drip, which causes small quivering waves to run over the surface of the water, is quite an efficient means of aeration, run at the rate of about one drop per second. This will use up about 1 gallon per day, so that in the average aquarium there need be no provision for overflow, but if this is desired, or if a faster drip is used, it is easy to provide.

The best device is the constant level siphon, which operates only when the water rises above a pre-arranged level, yet does not empty itself and become functionless when the flow ceases. The figure shows two types, both with essentially the same design. When the water level in the tank drops to that of the right-hand arm of the siphon, or of the hole in the top bend of the alternative design, the flow ceases, but it starts again when the level goes higher. The siphon may have a plug of glass wool or other device fitted so that  small fishes do not escape, but in actual fact they rarely do so, even when the tube is left open.

Instead of a drip, a fountain may be used, but this requires a lot of water. Nothing less than a flow of 40 to 50 gallons a day will be any good, so that a return pump is necessary to provide the fountainhead. Therefore the pump may as well be used in the first place as the primary source of aeration. However, in suitable surroundings a fountain looks very attractive. The tank must be large and the nozzle bore very small, so that a very fine jet is sent at least a few inches into the air, to fall back onto the water surface in small drops. The aerating effect is excellent, and the rate of flow of water is usually such that it all circulates within a few hours and comes into contact with the air in droplet form.

An electric fan playing on the surface of the water is also an excellent aerator, causing the water to circulate quite briskly across the top of the tank and then down away from the air current.

Pumping Water

If large volumes of water are to be moved, particularly from one level to another which is several feet higher, a water pump as such is necessary. For smaller jobs, involving a water lift of not more than a few inches or at most a foot, and a volume of not more than about 100 gallons a day (which is really quite a lot), an air pump may be used.

A stream of air is injected from a small-bore air line into a vertical water line of rather larger bore. This breaks the water and air up into bubbles, and they are carried upwards together. The air can lift the water some 4 to 5 inches above tank level, but efficient working requires a small rise only, unless the "Scattergood" principle is employed, or a rather complicated arrangement of tubing is erected beside the tank. Older water lifts used the air to push the water over from one container to another—usually from a tank to a filter carton or vice versa. In the Scattergood superflow the air is pumped into the water line after the water has siphoned over from one container to the second, as shown in the figure. This uses much less air under normal conditions and also permits the water lift to function even  if a large rise and fall is needed, as when the tank is half-empty, for the power needed to move the water is determined only by the difference in level between the two containers.

A useful size for the air tube in such instruments is 1/8inch, and for the water line 3/16 inch, both internal diameter. Both are commonly made of glass, but any other convenient substance may be used, as long as there is no danger of water poisoning. In ordinary use, such an air lift uses 5 to 10 times the air that is needed by an aeration stone, but the Scattergood type reduces this requirement back to about the same figure as the stone consumes, namely something around 2 cubic inches per minute.

It will be apparent that in any system of connected vessels it is necessary to install only one water lift operated as above, since siphon tubes may be used for all other connections. These tubes will ensure a flow of water such that all levels tend to be equal and will rapidly become equal once the pump is stopped. To ensure that a siphon does not cease to function, both arms must dip below the water surface, and in any set-up where the blocking of a siphon tube could be disastrous two should be installed in parallel. It is a much wiser precaution to arrange all systems so that such a blockage does not do any damage. Thus, a water lift from one tank to another should never draw water from far below the surface, so that should a return siphon become blocked, water in the tank from which it is being actively pumped will not be sucked dangerously low.

Filters

Filters use an air pump to shift water from the tank into a smaller container, or from the container back into the tank, the other part of the water lift being  by siphon. In the first case, a simple filter may be constructed from a glass or other container (plastic is easiest to work) by boring a set of small holes in the base and clipping it onto the side of the tank.

Water is then pumped into the container, passes through a bed of cotton wool, glass wool, or other material, and out again into the tank. When water is  umped from the filter back to the tank, the same design can be used; the water will pass up through the holes in  the bottom and through the filter. This, however, tends to raise the filtering medium, and the water passes beside it more than through it, and so the direction of flow is reversed by sucking water from the bottom of the filter, dispensing with a pierced base, and siphoning water from the tank into the top of the filter. The second type, which is much more usual, may be placed outside the tank, clipped onto the side or even on a separate stand. The siphon which carries aquarium water into the filter is provided with a "starter," consisting of a small rubber ball which, when compressed, starts the filter without the operator's having to suck at the tube, or to remove it and fill it with water. The principle is quite simple. There is a short tube from the ball which runs with the short arm of the siphon almost to the end. The ball is squeezed and held while the first finger of the other hand is placed over the two tubes at the same time.

When this finger is released the ball springs back to its normal shape and sucks water over momentarily, sufficient to start the siphon as long as there is only a short lift to be overcome, not more than 1/2 to 3/4inch usually.

It is common practice to place activated charcoal blocks or "coals" at the base of the filter, then a layer of glass wool, through which the water first passes. The glass wool removes any grosser debris or par tides, and the activated  harcoal adsorbs small impurities and also actively soaks up some dissolved material as well as actual suspended matter.

Both should be replaced at least once a week, and if this is done the filter rarely gives trouble. The water intake may be placed at a distance from the filter—perhaps in a front corner with the sand so sloped that waste matter tends to collect there and to be transported to the filter. Many filters have adjustable intake siphons to cope with differing aquarium depths and other dimensions. Others have more than one chamber, so that the water first passes through a glass wool bed and then separately through a charcoal bed, and perhaps a bed of crystals designed to adjust the pH.

Water from a good filter is beautifully clear, and the typical tank in which a filter is fitted looks as if there were no water present. It is rather questionable, however, whether this clear water is necessarily good for fishes, for it may contain a lot of undesirable matter in solution, even if charcoal is used in the filter. The point is that the filter not only removes mulm from the tank but also ensures that this matter is very efficiently bathed in a constant flow of water, which will dis- solve out anything that can be dissolved. Waste material which would otherwise collect in a still corner of the tank is constantly extracted and presumably yields more of itself up to the tank water than would be the case in an unfiltered tank. The word presumably is used because, to the author's knowledge, no actual investigation of this likelihood has in fact been undertaken, and no evidence is available to indicate that filtration is other than good for the tank. The possibility is mentioned because it exists, and because it should be worth while to find the answer.

It will be noted that filtration is also aeration, as the water is constantly streaming into and out of the filter. In a filtered tank, it is quite unnecessary to aerate in addition by any other means, unless with the object of stirring up and removing the mulm. A recent development is to use the sand at the base of the tank as the filter bed. This revolutionary idea seems to work, and adds much to the point just made, that filtration helps to dissolve mulm into the water. The air lift is connected at the base to a funnel-shaped pot which is pushed down an inch or so into the sand. Water is drawn into the sand around the base of the pot and sucked up through it into the narrow end by the air lift, and then discharged back into the tank. It need never be raised above the surface of the tank. The interesting fact is that the "gook," as one advertiser picturesquely calls the mulm, just disappears and dissolves away, eventually to be used (in part at least) by the plants.

Circulation Systems

The various devices so far described in this chapter may be used for circulating the water through a series of tanks, with or without the inclusion of a filter. It is unusual to circulate fresh water, but quite the reverse is true with salt water. The single salt-water tank is usually filtered and aerated by stones in addition, but when a number of tanks are run it is usual to circulate the sea water through these, through a separate large filter, then often into a reservoir,  which may be much larger than any of the tanks, and then back to the tanks. The water is best filtered before it joins the reservoir, and best circulated to the tanks separately, so that they are in parallel and not in series. In that way an individual tank does not donate its water, possibly infected, to another tank before filtration. The same principle may naturally be used with a single tank if it is felt necessary.

The danger of circulation is the spread of infection. The filter may not trap germs efficiently, and any infection may rapidly spread from the tank of origin to all the others. Yet, in marine tanks the benefit of circulation in other directions is so great that it is worth the risk, for the fishes may be in such better health that their resistance to disease more than makes up for this greater chance of their being exposed to it.

In fresh-water tanks, circulation has been used, on a kind of sewage farm principle, to demonstrate the biology of the aquarium, and the system has been adapted for fish-room use by a few enthusiasts. It is a complex arrangement, requiring batteries of tanks devoted to the disposal of waste by microorganisms and invertebrates, and aeration, filtration, and final recirculation of the water through the fish tanks. For class work in biology, nothing could give a better demonstration of the natural processes which occur in the pond or the healthy tank, but as a method for the routine purification of water it is a little too much, in the author's opinion, and also too likely to go wrong unless in expert hands.

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