This is a pretty niche issue so I’m looking for advice from people who preferably have experience with fancy goldfish. They are usually highly inbred and do often not adhere to regular fishkeeping practices

I have more and more fish that seem to get issues with mostly positive buoyancy (one with negative buoyancy).

I only feed with sinking pellets and always presoaked. I have tried raising the temperature, starving them for 3 days and feeding descale boiled peas.

I have also tried adding salt. Nothing has any long lasting effect.

I am now considering using yellow powder (funox) as I suspect bacteria to be the cause of it.

Out of 18 fish only a few swim with the belly up periodically. Many og the rest seem to be struggling with boyancy nonetheless.

Parameters seem ok. Ammonia and nitrite are always 0. Nitrate is maximum 20. I do weekly water changes of 50%-70% The water is pretty hard and PH is high but I think keeping it like this is better than trying to lower it and maybe exposing them to fluctuating parameters.

The only other issue I can suspect is that I use tapwater and not old water. There is not chlorine in our tap water so I feel it is safe.

What are your thoughts about my situation? Is my considered next step, of treating for bacteria valid? What would you do? I do not have access to aquatic vets and no access to microscope.