The water chemistry seems to be improving - the nitrite level appears to be around 1 ppm, down from somewhere between 1 and 3 ppm. Hopefully that means that Nitrobacter populations are growing, doing a better job of converting nitrite into nitrate. The alternative hypothesis is that the Nitrosomas populations have collapsed for some reason and the ammonia isn't being converted to nitrite. It's possible, but I think it's unlikely (though I suppose I should test the ammonia levels again).
On the other hand, the second largest male platy isn't looking good - he has splotches of on him, and he is being harrassed terribly by the largest male. I'm guessing the two are related. I need to get something to treat that...if I knew what that was. Anyway, I have a heater to return (it was too big for the small tank).
One real problem with trying to treat anything is that I have two things to manage - the fish and the tank ecosystem. Most anti-bacterials will destroy the biofiltration system, which will put the rest of the tank at risk. I really wouldn't want to start this whole process over. That said, of course, the health of the fish is important. A diseased fish means a much larger population of pathogens in the tank. Even though they were there to begin with, it's that reserviour that has the potential to infect the other fish, the ones that are less stressed.