Red Rio tetra (Hyphessobrycon flammeus) is big and chubby ... i had live bearers before so I'm used to fatties once in a while but this one swims a bit odd too. From what I read the main cause is bad water quality so I'll do a 50% change tomorrow (52 gal tank) just to be sure. Have been ill and less mobile so haven't been super diligent sadly, although I do use nitrate sponges in my filter and the tank is very heavily planted including a surface covered in floaters.

Apart from water change and being more diligent anything else I can do?

There are about 17 Rio's. Other tankmates are corydoras panda (7), kuhli loaches (rescues, 4) and a bunch of shrimp and snails. Apart from the occasional shrimp no deaths since ages.