is the one under treatment eating? looks like it was ammonia burn that progressed into a bacterial infection due to lowered immune system/increased toxicity. if the fish is eating or trying to eat then you are in better shape.
make sure the water is pristine, and add about 1 tbsp of salt per 20gal water. keep temp at 82F, and i would recommend 50% water changes every day until you start to see some improvement.
the damage is quite progressed, but the fish can still bounce back - keep us posted as to whether the infection is spreading, decreasing, or staying the same. we can advise on further measures if necessary. good luck; they are tough fish, so hopefully your little TPG will bounce back soon!--
--solomon