Thats pretty strange. The only thing I can think of is that the chemicals that you added did not sit with the fish well. That is probably obvious at this point. How often are you doing water changes now? I would be doing 50% everyday until he is looking better, and if you are going to use salt, 1 tabblespoon per 5g is not enough, I would go 1-2 teaspoons per 1g of water and if that concerns you just go with 1 teaspoon per gallon, but add it slowly and watch the fish for any negative reactions. One last thing it is probably not a good thing to add chemicals while treating with salt.