Belly up fish that's alive

[Fissssh]

I have a pair of "fancy" goldfish. Both got at the same time at the same size but one has far outgrown the other (about double the size). The smaller of the pair is now happily swimming round the tank upside down, so I've presumed a swim bladder issue and treated accordingly. The treatment hasn't worked. What can I try next?

out of interest

what was the treatmentt?!

Swim bladder often requires multiple treatments. Plus a filtered tank and regular partial water changes.

A liquid stuff from pets at home. You treat every few days with the carbon filter out. It's not worked though.

Also I've swapped to sinking foods not floating food, the tank cleaning regime is pretty strict and the water is well balanced.

Thank you. The tank is well balanced. Can't filter the water with a carbon filter while using the treatment according to the packet.

Apparently feeding cooked shelled peas helps. No idea why!