Fenbendazole works great for killing hydra. I had a hydra outbreak in my 5 gallon before I added any shrimp. After 3 days of dosing, there were no hydra to be found, even in my dense subwassertang forest. A word of caution though - it might kill some snails. I'd remove any MTS, nerites, or other snails you'd like to keep alive. Though the fenbendazole did nothing against my pond snails. Hope that helps.
RAOK Club #77|The Fraternity of Dirt #112