[Agu]

I've come up with two ways that destroy hydroids.

If it's a small colony mix up some epoxy putty and cover the colony. You'll probably miss a few so you'll have to do this two or three times.

If it's a large infestation a freshwater soak is the only thing that's worked for me. Notice I said soak not dip, basically you'll be killing everything on the rock. If you're lucky and it's confined to one or two rocks do this asap or your whole tank will get infected.