Hi everyone, I'm new to the forum.
I have recently had a strange occurrence in my 29 gallon planted tetra tank.
In the tank, I have 8 neons, 6 serpae, and 6 black skirt tetras (plus some otocinclus and a pleco). All fish have been in perfect health. A couple of days ago I did a 30% water change and added a small dose of flourish excel. I left for a few hours, and when I returned all the neon tetras had badly damaged tail fins, some were almost completely gone. The serpae tetras had less extreme, but new rips in their dorsal fins, and a few of the black skirts had small pieces of their anal fins missing.
I'm thinking one or more of the black skirt tetras became suddenly very aggressive for some reason? I have seen no signs of aggression from any of the fish, however. All of the fish are still eating well, and actively swimming as usual. No further fin damage has occurred since as far as I can tell.
Can anyone help me understand what happened?