Ok, it's hard to tell for me what may be wrong with your Oscar, but referring to your update I'd say he's definitely sick.
Did you check your water parameters - ammonia, nitrite, nitrate? If you didn't or if they are not fine (that means ammonia and nitrate 0 ppm, nitrate under 40 ppm), I'd suggest a water change and vacuuming the gravel as first aid - slowly, but at least 30%.
It could be ammonia or nitrite poisoning that causes the fish problems with breathing, or it could be a bacterial infection caused by stress or the feeders. Or a parasitic infection he caught from the feeders (that's one reason why many ppl here are against feeding feeders).
I'd take the feeders out if they're still in the tank, and add some dissolved aquarium salt or pure cooking salt (not table salt) to the water you replace, about 1 tsp per 10 gallon. It will help him breathe, and many parasites can't stand it. An additional airstone may help, too.
Before we are able to find out what your poor Oscar is suffering from, it's important he get's the best possible water conditions and enough oxygen.
It could help if you told us the a.m. water parameters, and perhaps send a pic. Is your sick O the only tank inhabitant, and if not, what about the other(s)? What size is the tank, and so on.
Sorry to have no better advice for the moment, but good luck so far. I hope he makes it.
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Gisela
moppel@gismo.owl.de Oops, I was to slow, but Joe is right.
Since you mustn't mix different medications and we don't know what it is he has, I'd first check the water quality and try the water change and salt method, though.
[This message has been edited by gisela (edited 06-15-2002).]