[LeTigra]

Hi
I have an 80gal with Tiger Barbs, Pictus Cats, a Rainbow Shark, some Kuhlis, otos and a few Harlequin Rasbora.
A couple of months ago I noticed the edges of some of my Tigers' fins were looking a bit ragged. I had 17 of them at the time. I googled it and it looked like fin rot or maybe some fungus infection so I treated with a finrot/fungus medicine. I then did it again a couple of weeks later just to be sure.
I'm doing about 30% water changes maybe once a fortnight - I don't have a strict fert regime so I don't do huge water changes and the tank is understocked and has plants. I don't test - the tanks been set up for 18months in this new house and for over a year in my last house so it should all be stable. But I am down to 11 tigers. No-one else in my tank is suffering - being that I have ottos, I figure if there was a problem they'd let me know.
One of my remaining tigers now has ragged fins. It looks like he has holes in his anal fin right close to where it meets his body. Another has a fresh "hole" in its side that looks like someone's taken a chunk out of him. And ANOTHER looks like he's had most of his pectoral fins chewed off and he's just wagging stumps

I don't want a lecture on not testing - I'm not the only one. I'm just looking for advice. If my tank wasn't stable my ottos would be dead. If I had a serious finrot or fungal issue surely it would show up on other species in the tank, right? I suspect that by morning I'll be another tiger down - he's not coping well with a hole in his side although no-one is bothering him about it now.
Are they killing each other? When I had only two I had less problems!