Over the past week or so, one of my two Corydoras's has been acting sick:  He swims as if drunk and settles down on the surface quickly or on the nearest plant.  Today he seemed near death (i haven't checked my 20 gal tank in the last 1 hr).  He appears pale. No growths are seen.  I have had him for nearly  9 mos. His previous partner was found dead over 6 months ago and I bought another who seems to be OK.  The other denizens appear OK, though one of my platy's had developed a swollen eye a week ago which has now cleared, and a few of the platys have died over the past several months (I am now on my third generation of these). Altogether I have 5 adult platys, about 8 or so juveniles with occasional fry added, an asian algae eater, 4 mollies and 8 neon tetras (the newest additions). The water is kept at 27 deg C (about 81 deg F) and the tank is heavily planted, with the plants supplying the oxygen. The CO-2, Ammonia, nitrites and nitrates all test "within normal limits".  I have a bacterial powered filter system (Eheim Professional) which seems to be working fine. I feed the fish daily with granulated food, and must admit that I haven't been generous with the additional tablets meant for the bottom feeders.  Could it be that the poor fish (who I've named "dopey" because from the start he would overlook the tablets when they were right by hm) has been malnourished and is dying of that?
I am grateful in advance for your site, since I am a novice aquarist, and found your site by googling Corydoras and disease.