Redox600

I posted a few days back (Fin Nipping) that one of my red guppys had its tail nipped and was acting very lethargic and hiding. But now the fish has died within 2 days. I had isolated him (red fish #1) in a breeding tank, but he still wouldn't eat and basically stayed still most of the time. I put him back in the tank with the other guppys and he instantly went and hid under the filter. The next day my other red guppy (red fish #2) was also hiding with red fish #1 under the filter and acting the same way.
So I came back to the tank to discover that red fish #1 has died, and red fish #2 has a tail that looks chewed up slightly, but I havent seen any other fish bothering him. I have taken red #2 out of the tank and put him in a simple bowl away from the 3 other guppies and the 1 cory I have in the tank.
The water parameters are normal, and I'm confused as to what happened... about 2 weeks ago I had a cory cat die, but had attributed it to a mini-cycle that I had not dealt with in time (ie high nitrites).
Is there a disease that looks like dorsal fin nipping? I'm now expecting my red fish #2 to pass away in a couple of days if I don't figure out whats going on.
Thanks for any help.

0morrokh

Were there any other visible symptoms aside from the fins? Several other diseases can cause finrot as one of their symptoms. I doubt any aggression between the gups would have killed one so quickly.

I've kind of given up on pet store livebearers--I've lost every single one I've tried to keep to some disease I've never been able to diagnose with 100% certainty, but always they grew lethargic, started sitting on the bottom, and then died, with or without other symptoms (one being rotting fins). I've tried treating with all kinds of things but never successfully. I don't mean to discourage people from keeping gups, but sometimes there's not much you can do once they start dying. Best thing is buy good stock from reputable sources.

For now definately quarintine any fish you see acting strangely. If you don't have a q tank that would be a good investment.

COBettaCouple

Parasites or bacterial infection are possibilities, but I'm not sure of either at this point. Parasites would seem to be the strongest possibility given the symptoms, but that doesn't cause fin rot so it may be both.

If you have vita-chem, fish protector or garlic guard - any or all of those could help. Parasite Clear would be what I'd try treating them all with.