Pond Boss Forums INTRO Questions & Observations Fathead minnows just keep dying

I am trying to keep flathead minnows alive in a 250 gallon tank to use as feedstock for Crappies until the crappie get to a size I want to release to a pond. I have a 1000 gal aerator, UV clarifier, two five gallon buckets for filtering charcoal, lava rock, bacteria balls, fiber filters, nitrate filter, one pump for filters and one pump just for water circulation and water temp kept between 50 - 60 degrees with chiller. I have added fatheads five times (2 pounds each time) I jut added 2 pounds Saturday and today about 70 fatheads were dead. What am I doing wrong? I have six crappie about 13 inches long that live in the tank, four 11 inch bluegill and two 9 inch bullheads and wild minnows caught from the creek do very well so fish can live in the tank. Everything but fatheads.