Ive raised these things on hotdogs before...no joke. Raising your water temp will help spur his appetite. Cooler water = slower metabolism. It will not hurt this fish at all to run your tank at 82 degrees, and he will eat like a maniac. Just raise the temp gradually, maybe 2 degrees per day. Make sure you have good surface movement in your tank so that theres plenty of oxygen in the water. 71 degrees is a bit cool for a fingerling. They are generally born in the early spring, so they hit the 2-4" mark right about the hottest time of the year, and they stay in very shallow water at that size. They need warm water when they are young.