maybe the condition of the water have change, like lack of oxygen, if the tanks have many fish it will lack oxygen.
I have similar fish in the same water..... 2 external pumps with 4 outlets for air supply and a big tank filter system that flows it back to the pool like a water fall.... and an independent submersed pump to circulate the water.
I saw something on TV where a deep sea diver used a syringe to expel the gas from the swim bladder so he could bring the fish to the surface with no problems. I'm just telling you what I saw. I'm not telling you to peform surgery!
Had a vet come to look at the fish while I was in the hospital.... pulmonary emphysema.... no cure... wont kill it as such.... but will be swimming upside down till it pops its clogs... poor little sod