The aquarium can be purified by raising the temperature to 100 degrees F for twenty to thirty minutes (no fish present) or by introducing Blue or Leeri Gourami fish which will eat the hydra when forced to do so by sheer hunger. Therefore, stop feeding them for a while. Some English water snails will also eat hydra, but they have the bad habit of eating plants as well.
Another way of eliminating hydra is to black out the tank entirely, leaving only a 2 in x 2 in gap on the side facing the light. Within a few hours all the hydra in the dark tank will have gathered on the 2 in x 2 in opening and can be easily wiped off with a sponge.
For the pond, one quarter of a grain of potassium permanganate can be added for each gallon of water. The dosage can be doubled if there is no fry. This should be left for two or three days and the water then gradually changed until half to two thirds of the original has been replaced. The fish need not be removed. Another alternative remedy for the hydra infected pond is ammonium sulphate added to the water at the rate of one teaspoonful to every ten gallons. This will destroy the enemies but not the fish.