People are quite often horrified when they first notice that their tank is inhabited by 'tiny white worms'. They could be one of three things: Hydra, Nematodes or Planaria. They can esily be identified by the way they move, and where they reside within the tank. Hydra are small carnivorous animals from the Phylum Cnidaria. They possess a simple cylindrical body with tentacles surrounding its mouth. The majority of Hydra reach sizes between 0.25 to 2.5cm (0.1" - 1"). They are usually tan or brown in colour, and are usually difficult to spot, unless your tank is heavily infested. They are usually confused with worms.
Hydra do not 'swim' per se, however they are capable of floating from one location to another. They will anchor to a spot where there is an ongoing supply of food, or they will just float around in the water. They anchor themselves to their surrounding environment by their 'foot', from where they catch and kill their prey. In the aquarium they can attach to things like gravel, vegetation, stones or filtration equipment.
Hydra prey on small crustaceans, worms, insect larvae or fry. Therefore they are usually associated with tanks raising fry. They are capable of killing fry from 10 to 15mm (0.4"-0.6"). Larger fry that are trapped but manage to escape will most likely die anyway, as an after affect of the Hydras stinging cells. Tanks that feed a lot of live brine shrimp can attract Hydra as well.
Hydra can be introduced into an aquarium by live food, snails, driftwood or water collected from natural waterways. A tank that possesses Hydra is not an unhealthy tank, as Hydra do not survive in poor water quality. However, a well looked after fish tank will not provide enough food to sustain large amount of Hydra.
If your fish tank has a Hydra infestation, salt is useful for the erradication of hydra at 0.3-0.5 % for five days. To reduce and hopefully eliminate Hydra numbers, water quality should be in top condition, which means 50% weekly water changes together with gravel vacuuming to eliminate residual food in the substrate .. and feeding should be reduced.
