definitely do good waterchanges, but be carefull of temp changes at that size they can't handle any difference in water temps. Keep the temps up, and medicate with melafix, and stress coat, do half dose on stress coat as it can coat the gills but the aloe is an anti-inflamitory for the covering of the gills. Put him on a strict diet of bloodworms, and make sure you bring the food to him if he doesn't seek it out. I use an eye dropper for one that size. Just thaw the blood worms suck them up in the dropper put it in front of the fish and squezz it out. He'll more than likely eat. They usually won't hunt for food if sick at that size. Hope this works. Hope you got a pond or tank in the works. I know a lot of people flame, but seriously they get large in a hurry. I've heard of 30" in two years, and mine is over 16" in less than 6 months.