A while ago my boyfriend and I took over his sisters fish as she wasn't looking after them properly. The catalyst was going round one day to find the water green, far too much food in the tank and the fish gulping at the top. We have since set them up in a more suitable tank with a filter etc but we have been concerned about one ever since and if it was potentially deprived of oxygen with what happened. He's always been a bit lopsided and never looked too healthy, we treated for swim bladfer which seemed to help slightly but this week he's really gone down hill. He's been somersaulting round his tank and doesn't seem to have much control over his actions. This morning we thought he was gone as he was lying under the bridge decoration on his side and not moving. As we tried to move him to a smaller tank it became clear he's still alive. I realise there is potentially little we can do but does anyone have suggestions as to what might be wrong and if it may affect our other fish?
This doesn't sound too hopeful and I hope I'm not too late with my advice.
However, what I suggest you do is very slightly/gradually raise the temperature in the tank, and keep the water as shallow as the filter will allow (obviously there will need to be room for his fins if he tries to swim). This will be less distressing for him as/if he tries to move and swim. A dose of aquarium salt might do the trick and help with some revival - it has worked for me several times when I had fancy goldfish in that state in the past.
[Although I don't think his condition will affect other the other fish, it's best if you're treating him, and adding salt or medication, to keep him on his own in this smaller tank that you've moved him to, provided that it has a filter. NB. If you have carbon in your filter, medication won't work, so best to remove the carbon element if using it. Also, make sure that you have some of the filter media (sponge, foam) in the small tank's filter, rather than new filter media as this will cause a problem with the water parameters/nitrates, etc.]
You could also treat with anti- internal bacteria infection, which also helped in severe cases like this, but, given that he's on his side and not moving, I'm not sure if it will help at this stage. I would definitely try the dose of aquarium salt - you'll need to work out the correct dosage for the tank he's in.
If he does perk up a bit with my suggestion, then I'd suggest the anti- internal bacteria treatment in a few days' time when he might be stronger and able to deal with it. I'd also recommend a change in diet to less flake/dried food, and the occasional frozen pea (with shell removed, and the pea chopped into tiny morsels), other veg (eg courgette - very small amount, chopped into tiny morsels), and live or frozen bloodworm which can sometimes clear intestinal blockages which can present themselves as chronic buoyancy problems.