My heterospilus had contracted callumanus worms from an unquaratnined pleco (i think) and the anus was swollen.
ZidanSanz
Does it look like anything is protruding from the anus ?
https://www.monsterfishkeepers.com/forums/threads/levamisole-worms-help-asap.645040/
https://www.monsterfishkeepers.com/forums/threads/red-swollen-anus.644956/
Not implying that this is what's going on with your fish but take a look at the two attached threads maybe something in there will pertain to your situation
https://www.monsterfishkeepers.com/forums/threads/red-swollen-anus.644956/
Not implying that this is what's going on with your fish but take a look at the two attached threads maybe something in there will pertain to your situation
