How big is the tank by the way?
And culprit could be anyone so it is possible its the platy, even if you put purely guppies they will from time to time will nip each other fins. Providing ample swimming space helps. Not heard nip till death yet though. But if the tank is small and there's a lot of fish, then ya, they tend to turn a bit more aggresive. Just like you and I. We all need our personal space.
Bookmarks