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Many writers seem to think that very frequent water changes substitute for the filter in a fish bowl. In some cases, particularly when you put to many fish in the bowl or over feed, this maybe true. However, biological filtration can also maintain high water quality without the help of electric powered pumps if the number and size of the fish and their feeding are kept within reasonable limits.
Bacteria grow in aquariums and fish bowls that change, ammonia to nitrites, and nitrites to nitrates. This is good because ammonia is toxic to fish, nitrites are less toxic than ammonia, and finally nitrates are less toxic than nitrites. Under gravel and other biological filters speed up this process, but it still occurs without their help.
For the bacteria to do their magic they need oxygen. This is one of the reasons for putting relatively few fish in the bowl. The fish take in oxygen through their gills and they also put out fish waste, ammonia, that has to be processed. So extra fish both create a greater need for a filter by adding waste to be processed and simultaneously reduce the ability of the bacteria to process it by removing oxygen.
In Exotic Aquarium Fishes Innes suggests that Guppy size fish need a minimum of about three square inches of surface area, and Swordtails or large Platies need about eight square inches. Innes suggests these figures be doubled or tripled for best results. These suggestions are based on aquariums that have neither aeration or plants. So this may give us some clue of the number of fish that would overload the ability of the aquarium to process fish waste without a filter.
I have over the last decade or so of fish keeping kept within the Innes limits. I usually try to follow his suggestion of having three times the minimum. So I would have nine square inches of surface area for a guppy sized fish, twenty four inches for a large platy. Sometimes I do crowd my fish more than this, but it is has been rare.
I have done quite well in keeping the fish alive under these conditions. Few fish have died and it is likely that none died because of bad water conditions. This suggests that the biological filtration is effective as long as one does not exceed the Innes' limits mentioned above.
I also have bought test kits for ammonia, nitrites, and nitrates. Numerous tests have never revealed a water quality that was really bad, and only rarely was the water quality less than ideal safe. The exceptions were cases where the nitrite level was to high in a bowl that I had just set up. With water changes I brought it into the ideal range, and with time it stayed in the ideal range without water changes.
I have thought about buying more test kits and doing a systematic study of how many fish I can stock, and how much I can feed and keep the biological polutants like ammonia, nitrites, and nitrates under control. Currently I am planning to finish writing what I can without doing a study like that.
I have found that when I crowd the fish, they seem to live quite well, but the bowls require more cleaning and water changing. Light stocking is a great time saver.
Deep Water Problems
Even without the problem of excess fish, in deep bodies of water that are too stagnant the deep waters can be depleted of oxygen. This is often mentioned in the literature on natural bodies of water, but it only happens at depths far deeper than a fish bowl.
I have read occasional arguments over the years that aeration maybe necessary for very deep aquariums. This might be the case even though even the largest home aquariums are very shallow compaired to natural bodies of water because there is no wind to stir the water. Nevertheless, this is probably not too critical in bowls, as bowls are usually shallow compared with aquariums.
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