I used the product intended for goats, its a 10% suspension. It didn't really work for my camallanus worms, but it killed the CRAP out of my snails. Lost all of my nerites and most of my MTS. Buddy of mine has hydra and I've given the remainder of the product to him, far as I know he hasn't used it and is still trying to kill them with H2O2 (why?)
You can easily find it at petco or whatever listed as Safe-guard for dogs, it comes in 3g (?) packets I think, maybe 1g. Whatever. Everything I have read says grind it up really fine or it will be a bear to dissolve, which is why I went for the goat liquid. If you have a farm supply store near where you live check it out.
Stay away from the horse paste. Hydra hate the taste of apple cinnamon.