You will want to get rid of the hydras ASAP, do not assume the fish are eating them. If the fish are biting them, they are helping the hydras reproduce. The hydras are dangerous as they sting and contain toxins. 1 or 2 hydras are not harmful but soon you will have thousands.
One of the following post has a link from someone who purchased Fenbendazole Panacur-C which successfully killed them.
I had an outbrake of hydras a few years back after I had bought a new plant. It was a disaster.
When I first seen the hydras, I threw away all plants and totally cleaned the tank putting in new substrate and filter. I tried to bake the tank decorations at 400 degrees for over an hour and the hydras came back after a few weeks. This went on for 2 months trying all sorts of things. I would knock the numbers down but they multiply extremely fast. Salt and raising the tank temperature will not kill them either. I ended up throwing everything out and starting new everything. Luckily it was a small tank with only a puffer so I did not loss that much money.
I would strongly suggest the Fenbendazole,