Im pretty sure they will ever over run a tank if you was to overfeed consistantly. Do you have and fish fry or shrimp in the tank?
Ok, them hydra are pretty big. Have to be careful because they can pick off fish fry and baby shrimp. Pond snails will actually eat hydra.
Try the sticky thread: http://www.ukaps.org/forum/threads/bugs-you-might-encounter-in-your-aquarium.35994/#post-390760
The battery method seems to work well.