It is never a good idea to have such wide variations in temperature in your aquarium. Large temperature fluctuations weaken your fish's immune system and leaves it vulnerable to disease. This is why aquarium heaters or a steady room temperature is very important.

It is good that you have been feeding and cleaning the water appropriately. I am going to assume that you have been using the right water conditioners etc. Right?

In any case, it sounds like your fish has developped some type of infection. Check him over with a magnifying glass to make sure there are no parasites attached to him. You may then change the water every 2 days (proper temps and conditioning are mandatory for the water) and add some aquarium salt, 1 tsp, to the tank water. You really should try and keep the temp of the water steady. 25 degrees celcius (77 degress F) is the least you should keep him at. If he does not start to perk up within 1 day or 2 for the most. Then use some medication like Maracyn or other type of medicine for bacterial infection if he has no signs of parasites. That's the best I can do. But, in any case, make some water changes and add aquarium salt (do not overdose on salt as you will kill the fish). Good luck.

Dasheen

Also, it sounds like you may be over feeding. I would cut it to once per day or even every other day.

-PF