I just saw this today... the cardinals all seem fine except this one. Can anyone tell me what this is, and if I need to treat? There's been no new additions and all the cardinals came from same batch at LFS.
Thank you so much! Since this is my first fish I want to make sure I don't miss something stupidly obvious!Agreed that it's most likely poop. Keep an eye on the fish though. If the poop starts to move, the fish constantly has poop hanging, or the poop is stringy, that may be a sign of internal issues.