if theres both an amphipod and a small worm, that would indicate that they are only cleaning up something dying. if it was just amphipods causing it to die, then you have a species of predatory amphipods. at this point the only option would be to interceptor the whole tank, but im pretty sure you're not wanting to do that.
i treated my system for another reason and everything turned out fine. after my treatment though, i didnt see the amphipods that once use to nip at my zoas before hand. other amphipods have bounced back