Thanks for answering the questions and including a photo. We all learn from scratch at some point, and sometimes we were fed the wrong info (esp. by pet store staff) and sometimes we were careless, the only thing we can hope for is that we can try to reverse the damage we have caused. Don't beat yourself up over this, the only thing we can do is move forward
Initially the tank size was 2 gal and now you have moved him to a half gal, just think of it as putting him in the hospital tank. The only problem with either one was the water change schedule. Just for your future reference, a 2 gal should have at least a 50% water change midweek and 100% wc end of week. (I personally would do 2 100% wc a week). As for the half gallon, since he is now sick, I would probably do 100% wc every one to 2 days, every day if doing treatment. They should be fed every day, 2-4 times a day (based on your schedule of course). Temperature wise, ideally should be 76 to 80, and most importantly it should not fluctuate by more than 2 degree to avoid shocking their system.
So for now, change his water daily. Get a heater that is adjustable. Try to adjust the heater by slowly increasing the temp, ie 2 degree per day until you reach 78 or so. Are you able to get some frozen blood worms or brineshrimp? They like these type of food. You need to help him keep his strength up which means he needs to eat.
I am sure others will come along to add to whatever i have missed. Are you able to take a close up (upright) photo of these black spots?