Looks like it to me, but I'm no expert. The good news is that it is chronic, and not immediately fatal, or so I have read. By now, you've probably read 20 articles and have some idea of how to proceed.
I'm not really certain what to do. He is very lethargic, but then he and the girl will speed around the tank and wiggle at each other, and then he'll be lethargic again. he was not like this before I left for Paris. I am toying with pulling him and putting him in a little tank and treating with Metro.
I'd try mixing up the diet a little, too. Maybe some live or frozen food. Maybe even see if you can find vitamin supplements at Congressional Aquarium.
Water quality can be a factor, or so I understand, but I didn't suggest that because I know you are such a conscientious aquarist. Your point about the decomposing fish makes sense, however, so I'd keep the water changes going for a while.