I seem to be having a problem with my big tank as I have lost 4 out of 7 spotfin shiners, 2 blunt nosed minnows and a crayfish over the past 2 weeks. My nitrates were quite high in the tank and I have been doing water changes about every 2 days to reduce the nitrates. Other parameters have remained stable with 0 nitrites, 0 chlorine, 0 ammonia and the water being the same hardness, acidity/alkalinity and pH as they came from originally as I use only source water that they've come from. So far the only fish affected have been the shiners and blunt nose minnows. The darters and N Longear sunfish seem to be fine, active and eating. Any other ideas on what the problem might be? I did a massive water change today replacing 80% of the water I changed 2 days ago with fresh native lake water. Is there something that I am missing here?