European Giant

Hi, I treated a tank for hydra 5months ago with fenbendazole (dog dewormer). I know it's lethal for nerite snails, so they've been living elsewhere for a while. Does anyone have any experiences of reintroducing nerites to a tank post fenbendazole and any advice on how long to give it before adding them and anything else I should do to the tank. Thanks!

This hydra looking, anemone looking being appeared about a month ago in my fry growout tank (60L or ~15us gallons) It came out of nowhere and exploded in numbers and I've been battling it ever since. At first I dosed it with fenbendazole at 1ml of 10% solution per 50L. It had no effect. I left it a week and nothing happened unlike normal hydra which normally dies quickly. I have since tried no planaria using the manufacturers guidance and again no effect! It costs everything in the aquarium. The glass, the plants, the filters to the heaters and it is entirely resistant seemingly! Does anyone know what this is or how to treat it? I think I may have to bleach the tank. Many thanks