What is wrong with my guppy?

I've recently had a spike in nitrite, not to super concerning levels, so I did a small water change (mistake). Some of my guppies sadly got finrot. I treated them and most got better. One got a lot worse. He is VERY bloated and has some sort of growth on his fins. What should I do? Is there even a way to fix this? Some of the time, he is away from the others, sometimes swimms with the group. Has no loss of appetite.

First ANY, even small, a spike in nitrite level is something of concern.

You treated with what? Did you have a clear and definite diagnosis of the issue to warrant the meds? Many times meds can cause more harm than help if not proper for the situation. Sorry to be crude but it can be like taking a laxative for a head ache. You just don't do it. It is the same with a tank.