they are robust fish, salt is good and freshwater is great. Its a small tank so frequent water changes should be in order. Use water that has sat out and been treated for chlorine's and such. Melafix may not be a bad idea. The temp is important and if possible I would recommend a new heater if you cant not get to high seventies or low eighties. Watch your water parameters, if you just set this tank up and are using a new filter your ammonia and eventually your nitrites and nitrates may become a problem. your fish is more than likely going to have a heavy respiration rate and with the higher temp the water may foul very quickly in an un-cycled tank. the result would be more stress on an already bumbed out fish. If you keep the water clean and good food coming in you will be giving him the best chance.