do his spikes look bigger then normal? Does his stomach look bloated? A 20 gallon tank for a qt can stress a fish out. Especially if it is 5 inches. I used greenex in my tank, and the spots were less noticable. You could try that. Do you have anything in your qt? If you want you could try that. I used it in my display tank once, and it is invert safe. It will turn EVERYTHING green. It is up to you if you want to try that, but you don't have to. I did it as a preventative measure, just in case it was something besides stress. If his stomach is puffier then normal, then it could be a bacterial infection. It does not sound like that. If you do not keep anything in your qt, then when you added the larger puffer, your tank might have had a mini cycle, due to the HUGE amount of waste puffers make. IMO, if he has not gotten worse, or better, and he is eating and swimming, then I would NOT treat with antibiotics, and add him back to the main tank. What are the levels in the main tank? Puffers are messy, they make a huge amount of waste. You should try to keep nitrates below 10 for a puffer. Also (something to think about), do you feed your puffer until it gets fat??? Puffers can develop fatty liver disease. This is caused by the overfeeding of the "always hungry" fish. I had a 5 inch puffer, and i fed him 6 krill 2 times a day. Once I started to feed less, my levels in my tank improved alot. Let me know how it goes. If you have any other questions just give a shout!!!
HTH
Leopard