In all aquaria it is only a matter of time before unsightly debris collects on the surface of the gravel. Periodically this accumulation reaches a point where it is necessary to remove it before it causes problems. Aquarium sanitation is very important, because this unwanted debris is the breeding ground for bacterial colonies which are in most instances responsible for diseases.
Red-Rim MelaniaMelanoides tuberculatus
Photo source: cars.er.usgs.gov
There are at least two major school of thought on the use of scavengers in aquaria to prevent the accumulation of organic debris. On the one hand, some hobbyists prefer the use of snails, for snails will devour any dead matter short of fish feces. They greedily devour uneaten fish food, dead fish carcasses and decomposing plant matter. Furthermore, it can be said with little resevation that snails, at least the types commonly available for use in aquaria, do not eat even very tiny fry.
There are, however, several problems with snails, especially ramshorn snails (Helisoma spp.), pond snails (Lymnaea) spp. or the hard-shelled Melanoides species. They reproduce at such a rapid rate that it is sometimes difficult for aquarists to control their numbers, and if there is not enough organic matter for them to feed upon, they will devour flourishing aquarium plants. Both of these problems can be controlled somewhat, but it does require some extra effort. These snails often show a preference for lettuce leaves over the leaves of aquarium plants. Floating a small piece of lettuce on the suface of the water can supplement their dietary needs sufficiently so as to reduce their predation on aquarium plants. Furthermore, since they tend to accumulate on the lettuce in large numbers, the lettuce can be removed from the tank and discarded, thus keeping the snail's numbers in check.
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Photo source: zierfischfreunde-ih-hh.de
Most of the problems with snails can be solved through the use of mystery snails (Ampullaria spp.), which are quite a but bigger than those mentioned above. In addition, they lay eggs in masses above the waterline, so the eggs are easy to find and destroy. Some hobbyists also feel that these snails do not destroy flourishing plants as readily as do some of the others. Be careful, Ampullaria snails are known to carry several types of Salmonella bacteria.
On the other hand, some aquarists prefer to use catfish as scavengers in their aquaria. Experience has proved that the best catfishes to use for this purpose are those belonging to the genus Corydoras. These peaceful, small catfishes adapt well to a wide variety of aquarium conditions and do not often bother even newborn fry. They constantly dig through the surface gravel, rooting out small bits of food that the other fishes have bypassed. while they readily eat uneaten fish food, they are not as fond of plant debris as are snails. Like snails, and in spite of rumors to the contrary, catfishes are not feces eaters. Although Corydoras catfishes do not usually eat living baby fish, there have been cases in which such predation has been noticed. (Adapted from Greg Jennings. 500 Freshwater Aquarium Fish: A Visual Reference to the Most Popular Species)