Join Date: Mar 2011
Location: Oregon State Beavers!
I don't know if there is a formula, but I use two little 12 ounce Gatorade bottles in my 40b reef. I just freeze them with salt water (lower freezing point, I think) and drop them in the tank on really hot days. It'll drop the tank about two degrees which is enough. Then I just refreeze the bottles. I use the little ones simply because they are smaller and don't smash things around in the current.
I've also heard of people making cubes out of ro water and just dumping them into reef tanks. Using too much air flow over tanks causes excess evap which means more top off, so I prefer the frozen water method. Hope this helps.
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