Serpae Tetra acting sick

His belly seems kind of swollen and he is swimming with his head
up, tail down. He can't seem to swim normally. All the other fish in
the tank look fine. I did a water change, rinsed the filters and added a
little salt. Does anyone have any idea what could be wrong with
him or what else I could do? Thanks in advance.

Thanks for your reply

My water is good. The tank is a 29 gal. The fish in there are, 3 serpaes,
3 Tiger Barbs, 3 Rainbow fish, 2 cories. I have a hob filter and a large sponge filter. This tank has been set up for several years. I do a water change once a week. I rinsed the filters in the water I took from the
tank. Normally I don't use any salt, but I put some aquarium salt in
after the water change, thinking it might help the sick fish. I hate to
add any meds, not knowing what is wrong with this guy. No new fish
have been added for months. The serpaes are a couple years old.
Any suggestions appreciated.