Red Cyst On Guppy Fin

hi guys, i have found one of my guppys with a red cyst like angry growth on its top fin, he seems happy enough but has anyone any idea what it is or remedy, thanks

Is it a red lump or a red patch? If it is a lump, I don't know what that could be I'm afraid. But a red patch could be from nipping or catching the fin of something - quite easy for fish with long fins.

The best thing to do for it initially is more than usual water changes and keep an eye on it. You are watching for the fin being eroded; white growing on the site; bloody streaks in the fin. If any of those (or anything else) starts to appear, we'll know what to treat it with.