I recently got a new fish a couple of weeks ago that i've had in quarantine. He's in a .3% salt right now, and I have fully treated him with Prazi because at the store I got him from the fish always seem sick and they have an anchor worm infestation right now. (I noticed the anchor worms at the store after I had bought my fish, but I knew I shouldn't have gotten a fish from there.) He doesn't have any achor worms on him, so I think he's clear from that. But I've noticed that on one side of his body his abdomen is bulging out a lot more than the other side and you can kinda see his insides or something through the bulge. It looks kinda bronze inside? Is there something to worry about with him? Is he sick? He eats normally, but he is not particularly active. He just kinda swims around lazilly pretty much. Ammonia is at 0. Nitrates are at 20. Nitrites are at 0. Ph is at 7.5. It's a 10 gallon tank with only the one small telescope goldfish. See pics below.

Its hard to say with lumps and bumps what the cause is, but usually they are tumours. Most are benign and dont cause the fish any trouble but some grow in the wrong place and will eventually kill the fish.
For now just keep an eye on it and hopefully it wont grow with him.
He really is a pretty fish.

Some fish just aren't formed perfectly symmetrically. I have a few lopsided fish who have been that way since I got them and so far there hasn't been any trouble with them. As Sandy said, just keep an eye on him.