Change the water often, increase water temps to summer water temps 78-82, and feed moderate amounts of high protein high vitamin foods two to three times a day. I don't feed as much as a fish will eat in a couple minutes with the high protein foods. A couple krill, shrimp, earthworms, night crawlers, gut loaded crickets, gut loaded grasshoppers at a time. You only need a couple but it is more crude protein then a staple diet.
A coldwater fishes primary drive is to get as big as it can during the spring, summer, and fall so it can last the winter on its reserves and not be less of a bite size morsel. If winter never comes then no need for reserves but the fish will still have put on the size for the several weeks it was doing this. Then you back down to feeding staple once or twice a day during a winter.
Dirty water, cold water, and too much staple makes a fat stunted fish.