Sorry about the lack of details earlier...
20 gal tank
all the perams are good (nitrates 10, nitrites/ammonia 0, ph 6.8)
it's got a large internal wisper filter
I feed flakes, brine shrimp, blood worms
The origional sickness came in with a corie but the symptoms that killed all the other fish seemed like neon tetra disease or a type of bacterial infection. The fish would suddly get pale/a little bloated and be dead the next day, a few didn't even show signs of illness but were simply dead - it killed about 10 fish in a week.
I've had my blackskirts forever and they've never been ill, but lately they are hiding constantly and avoiding one another. The one I noticed has a small white spot on the tip of his tail and a tattered fin. It could be fighting damage, but it seems a bit odd. I think he may be pale as well.
I do have a quarenteen tank and I may put him in there (if I can catch him!!!) but I'm really afraid the bacteria/virus is still in there. I know how weak melafix is, I used it in an attempt to maintain my cycle but at this point I'm willing to use something heavy duty
Chrissy