That does look nasty. The only thing that immediately jumped to mind was the feeder fish, as it might well have nipped some of the slime coat from his body. I would recommend removing it from their tank unless you can see that it has been caused by something else.
To keep my own tank cool I use a combination of fans (desktop fans) positioned towards the water and held safely in place by the tank lid. I also fill old bottles with tank water, then freeze them overnight (I have two so there is always one spare). I then take the lid off the frozen bottle and, as the ice in the bottle melts, it sends cold water out into the tank. When it's empty I just fill it straight from the tank and freeze again. Some people also drape damp towels around the outside of the tank glass (obviously, be careful of plug sockets when you do this). The ultimate is a chiller - I can't afford to buy one of these, nor am I creative enough to make one, but there are lots of DIY Chiller options made from hosepipes and mini fridges on here and Google.
I would personally take him to a vet. If he was mine, I would also take him out and put him in a fresh hospital tank (just a warning; that hospital tank will need to be water changed once daily, more if he poops, with fresh, dechlorinated,aged water). I would feed him worms, and tea bath him (unless you can see fungus).
To tea bath, take any regular old tea bag (not herbal or fruit). Make up a cup of black tea with hot water and then wait for it to cool (if you want you can put it into a fridge). Wait until it hits room temp and then pour it into the hospital tank. You can keep the same tea for the next day if a whole cup is too much (but please don't keep it in a pretty white china mug, as it will stain it). You will need to replenish this daily after doing full water changes.
This should speed up healing, although he may well pine for his mate. If you can't bear to separate them, you could add tea to their main tank, but I tend to use a hospital tank simply because the fresh water seems to speed up healing.
Hope this helps!
