The hydra is only dangerous to very little fish. It is a low type of life, a polyp, that devours fish up to about 3/16 -in. long, and also daphnia.
It has from 3 to seven tentacles spreading from the mouth in a star-like formation, to which the thin elongated body is attached. The actual shape isn't the exact same in all specimens. They differ widely, making it nearly impossible to describe them with out going into volumes of detail.
It attacks by waiting for a fish or daphnia to pass near sufficient to be caught by the tentacles.
It is then drawn into the mouth to be digested - undigested portions being returned via the mouth. It is not uncommon for a single hydra to have daphnia held by every tentacle waiting their turn to be devoured.
They multiply themselves by budding and division, and given great conditions propagation is rapid.
You are able to get rid of this pest by removing all your fish and bringing the temperature of the water up to 105°F. This is rather an inconvenient technique, and you might prefer to add one teaspoonful of household ammonia to every five gallons of water.
Permit this to stand for two hours after which a complete change of water should be made in the aquarium. The ammonia won't harm the plants, but fish should be removed.