It's a 100L tank, parameters are all fine, in my tank I have guppies, pair of sword tails, pair of mollies, albino catfish and a bristlenose pleco,
I did have a kribensis but he was way too aggressive (kept attacking the guppies) so I had to take him back, would this stress have caused this? this guppy is one of my newer ones (few week old). I do weekly water changes of 30%. I feed them flake food. It's just this one particular guppy I have seen this on, noticed it a couple of days ago.