Okay - here is an update:
I've given the tank a 75% water change, added conditioned and salted water (about a fourth to a third of a teaspoon for a gallon), and have put the carbon filter back in to the system. I took another reading and the ammonia level was at first .25 then .50 on the next testing (I took another test right after the first one after changing the water). Extremely worried, I put in 3 gallon's equivalent of Prime water conditioner (7 drops), as I read on the container that up to 5 times the normal amount can be applied to the tank directly in emergencies.
What I am thinking is going on is that the tank never truly cycled to begin with, something I didn't really understand at the time, and the ammonia levels are spiking since I did not change the water as usual due to my fungal clear medicine directions. I would really prefer the tank to be cycled, so I looked up some info on it on the Tropical Fish forums, and it is said that I should just continue to detoxify the ammonia with Prime and maybe even add some extra starter bacteria (supposedly the Bio filter I received with the tank had some bacteria on it, but maybe it didn't actually have any or didn't have enough to deal with the ammonia.
I have also cut back on his feeding - only three or four pencil-point-sized flakes a day, maybe even less now.
What is really concerning me is that he is still behaving erratically - he will swim around like he normally does, but then just... stop. A little gill movement, but no fin movement at all, no up or down motion. Like he has frozen in time. When he does swim, he often has difficulty keeping his body straight, difficulty swimming downward, and his tail end will sometimes drift upward, so I am thinking that it is a swimbladder problem. He will also drift up to the surface and be on his side for a number of seconds, then go swimming off again. I have to say though that he is not as sluggish as he was before I put in the conditioner when he is active. He will probably stop in place four times every minute. *sighs* I know he is my first fish, and I accept any errors I have made, but still... I really hope he pulls through.