Zebra Danio swollen belly

[tetra45327]

Hello, One of my zebra danios has a very swollen belly. It has been this way for a few months. He still swims around rapidy, eats and chases other fish. The anus area is a bit pink. Also the spine is curved vertically meaning not side ways. The bend on the spine is at the dorsal fin. I think the water quality is fine. PH is 7.4 or a little above. nitrates are 10ppm, nitrite and ammonia are undetectable. The tank is 55gallon breeder half full and heavily filled with emersed plants. the filtration is two fluval 304's cleaned very two to three weeks with no carbon.
Any advice at all would be welcome.

Thanks