hi pigeon,
won't frighten the fishes. do this fast. "serok" the fish, close his eyes with your hand so he won't jump.(thats the secret when you net your fish. always close one of their eyes to prevent them from "loncat-loncat"). use a scissor can too. but not trim the hard fin(the front). soft fins(back fins) can grow perfectly but need time. that can be cause by "jamur" (how to say this in english, stan?) better don't use acry but methylene blue will be better.