Although preventing disease or parasites that exist physically on the fish is usually the aquarists first concern, the aquarium occasionally attracts other unwelcome guests. The most common aquarium pests are snails, Hydra and diskworms and following paragraphs aim to explain why these animals are considered a nuisance and how to control them.

Snails

Usually imported in to the aquarium by new plants and occasionally by live foods, snails themselves pose no threat to fish. In fact some snails are actually desirable in the aquarium. The exception to this is the Wandering Snail (lymeaea ovata peregrai) which produces a poisonous substance that can cause convulsions in fish.

Apple snails Ampullaria sp., which can grow to the size of a small grapefruit, are often purposely introduced as part of the aquarium display whilst Trumpet snails, with their characteristic "cornet" shaped shell, burrow through the gravel turning it over introducing oxygen and prevent wastes from clogging. Unlike their more common relatives the Ramshorn snail and Pond snail, the Trumpet snail does not harm plants and is often welcomed by the fish keeper as a sign of a healthy aquarium.

Snails usually emerge at night to forage on left over food and graze on algae therefore, a small colony should not be of concern. However, prolific breeding characteristic of all snails quickly translates in to excessive populations with a limited food supply and their attentions soon turn to destroying plants and over running the tank. Additionally, as with all living creatures, snails produce excreta and thus large colonies of snails can result in quickly deteriorating water quality.

Snails can be prevented from being brought in to the tank on plants by bathing the plants in Potassium permanganate for a few hours although once introduced, snails can easily be removed from the aquarium by a number of means. The simplest and most effective method over the long term is to keep a snail eating fish such as the Clown Loach (Botia macracanthus), the very effective Talking Catfish (Platydoras costatus), the Spot Gourami (Trichogaster trichopterus), or one of the many Pufferfish varieties.

Using fish to consume snails has the added benefit that the snails are actually removed from the aquarium rather than killed and left to decay. As snails have a high protein content, dead snails decompose rapidly at tropical temperatures and can quickly poison the water introducing other diseases. It is for this reason that I advise against using chemicals to control snails as complete populations can be left to rot at one time. Additionally chemical treatments often contain metal salts that are neutralized by water conditioners. It is therefore not uncommon for snails to emerge even after the application of a strong chemical dose.

An alternative means of controlling snails is to limit their food. Snails thrive on wasted food and can be a sign of overfeeding your fish. Additionally snails can be removed by hand. This is best done half an hour after lights out when the snails emerge to feed. Persistence is the key to this approach as inevitably, one must remove as many adults as possible and allow an eggs to hatch and mature before the tank is snail free.

Hydra

Hydra, small white worms with feathery tentacles, are introduced by live foods or on plants and like snails are not harmful to adult fish. Hydra do however, thrive on fish eggs and trap fry with their tentacles and thus must be eliminated from the breeding aquarium.

Hydra can only survive on live food so they can easily be controlled by changing to a diet of flake. If your fish cannot cope with this, a careful dose of 0.5ppm Copper Sulphate will do the trick. Copper Sulphate is toxic to fish so this treatment must be a last resort and used with extreme cation. Bear in mind also that some water conditioners have a high metal ion binding capacity and may render a Copper Sulphate treatment useless. Alternatively, again assuming that you fish can cope, a raise in temperature to 108°F for four hours will kill Hydra.

Diskworms

Diskworms are light brown worms that like snails, become prolific if you have a tendency to over feed your fish. As with Hydra, a treatment of Copper Sulphate is an effective solution but the best method is to introduce fish to eat the worms. Gouramis are adept at this. In particular the Blue Gourami (Trichogaster trichopterus) the Pearl Gourami (Trichogaster leeri) and the Paradise Fish (Macropodus opercularis) are recommended.