On a very hot day 107 F and higher I use those small square cooler blocks you can throw into a back pack or what ever. They are blue and last forever as long as you do not puncture them. You just refreeze them use again and rotate them. I place them in the appropriate size zip lock baggie and place them in the sump compartment. I also raise the lights and make sure the fan is blowing across the surface of the water.