I don't see why you'd want them gone, they're fascinating creatures. Pond snails are known to eat hydra, and would probably be fine with your arowana. Of course then you might end up with a snail infestation.
Hydra are notoriously hard to kill. They're called hydra because, if you cut one up into a hundred little pieces, you end up with a hundred baby hydras. I don't know exactly which type of "medication" would work, but be careful with such mixtures, as they can do more harm than good. Lots of weird stuff out there.