I've lost 2 fish in the past from the same problem. My last one was a wrasse that ate like a pig but slowy wasted away over the course of 4-5 months. It was a pretty sad thing to watch. Both times I tried feeding lots of garlic, which is supposed help with internal parasites, but it did not help. There is a product called Focus made by seachem which is a polymer that bonds medications to food. You could try mixing focus with metronidazole (Flagyl) and feed them the antibiotic. I wouldn't recommend treating the whole tank with with metronidazole since it probably won't be very effective with internal parasites.

Of course it could be something other than internal parasites but since it seems to be affecting more than one fish I would probably try to treat them. Sounds like you are feeding plenty. When I get a chance I'll break out my super duper fish disease book and see if there are any other suggestions. If you want to try this...Hex-A-Mit by aquatronics is metronidazole and I'm not sure if anybody around carries Focus - we don't.