Alaris
Aquarium Advice FINatic
I searched for hydra and found a few articles with consistent treatment suggestions.
Hydra
Good luck, Dottie. Hydra are freaky scary.
Hydra
Hydra can be introduced to the aquarium with live foods, snails, driftwood, plants, or water collected from natural freshwater areas. Most hydra in a normal aquarium go unnoticed, but within the confines of a small fry-raising tank, these little pests can be deadly, and can ingest a tankful of newly hatched rainbowfish larvae in less than a week.
Hydra only seems to appear in fry tanks being fed brine shrimp nauplii or similar live foods. They don't seem to appear in fry tanks that are fed primarily a dry or liquid diet. However, once you start feeding large amounts of brine shrimp nauplii, it isn't long before hydra appear, often in enormous numbers.
Flubendazole was mentioned in other articles I found about eradicating hydras. The articles also say to stay away from any treatment with copper (you already know this) and any suggestion of electrically shocking the water (don't know who would be crazy/stupid enough to try this to begin with) and that removing the livestock and heating the water to 40C and sterilizing with chlorine is effective, but not practical (esp for you).Dactycid (a product available in Europe) is an effective treatment against gill and body flukes, and other types of internal worms (hookworms and roundworms) including Camallanus cotti, hydra and planaria. The active ingredient is flubendazole.
Good luck, Dottie. Hydra are freaky scary.