ice packs. You can buy plastic ice cubes that fill with water. There meant for drinks, but they work great. Do some experiments, like (X) amount of cubes will cool the tank to (X) deggres, for (X) amount of hours...ect
75 gallon, fillstar xp2 with inline reactor, 12 Lb pressurised co2, 4.2 WPG-corallife 3x96 watt PC-, fluorite/silica-sand substright.