Whats wrong with my cardinal tetra

[Rinzy]

One of my cardinal tetras is all puffed up around its gills - its very red and it kinda looks like the texture of the cap on an oranda goldfish. He still eats normally and schools with the other guys. Any idea what is wrong with him?

 

[Rinzy]

This is an established tank
30 gallons
0 ammonia
0 nitrite
25 nitrate

 

[GNarnian]

What are the other tankmates with him?

 

[Rinzy]

One dwarf gourami
One male swordtail
3 oto
6 cardinal tetra

All the fish are cool with eachother. There's no chasing or fin nipping

 

[GNarnian]

Someone else could chime in here, but usually the most common cause of red swollen gills is high levels of ammonia. What do you test with?

 

[Rinzy]

I use API master test kit. I know what ammonia burns look like. This is more like a growth like a tumor or something

 

[Rinzy]

Anyone?