Re-circulate, re-circulate, re-circulate. The more fish in the well the more often I re-circulate. 5 fish use the available O2 5X faster than 1. I also carry frozen water bottles in the warmer months to help try to keep the water cooler. Also, since warm water has less O2 capacity than cool water I will use an aerator when water temps get warm and I will use fish dope. However, I think the key is to keep pumping fresh water.