Need opinions on what to do with seems to be I'll neon tetra

[Jensendani]

I've had my neons along with someother fish for a while now. All levels in the tank (20 gallon long) are good. But one of my neons seems to be really bloated and it won't go down. I didn't think much of it and just thought he had been eating a little too much. But now it's getting worse to where is eyes are puffing out. I also noticed today that his stomach looks like there is some red splotches underneath the silvery part. Any advice?