Put him back in his old tank and do a 75% water change and gravel clean the substrate every day for 2 weeks. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Drop the water level down so it is about 4-6 inches. Make sure the filter and heater still work.
Don't use Melafix, Pimafix or Betafix on labyrinth fishes (Bettas & Gouramis), because it can leave an oily film on the surface and cause the fish to suffocate.
Most fish health issues are caused by poor water quality or a dirty environment. The following link has information about what to do if your fish gets sick. It's long and boring but worth a read.
https://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/