I know this post is old, but having run into a major hydra infestation myself, it popped up on a search I did. Just tried some Fenben today, guess I'll see how it works.
Meantime, I learned there is at least one drug free way to kill hydra, though it only works if the livestock can be removed for a few hours time. If that's possible, using water heated to 104 to 110 F for a couple of hours kills them all stone dead. My infestation came on a new plant, and when I realized how many there were ( green ones, they look a lot like bits of algae from a distance), I pulled the plants out and treated it in hot water. Worked very, very well, no trace of the hundreds of hydra remained afterward. They were a few different Aponogetons, including Madagascar lace.
Since I could not find a heater that would raise the temp that high, I put the plants in a bowl and started with water at 110F, and wrapped the bowl in a towel to retain the heat. I tested the temp periodically, and when it dropped below 104, I topped it up with more hotter water to bring it back up, no higher than 110 F. After a couple of hours, let it cool off to room temp and no trace of hydra.
This could be done for a tank, with plants, decor, etc. provided the livestock could be removed for the time necessary. I think I'd take the filter media out too, just in case it's too warm for the beneficial bacteria to tolerate.
