Iam setting up a room for all the tanks soon. I will be putting in a 1.HP split system reverse cycle air con. It normally don't get this hot for more than a few days a year. I could slap up a chiller in no time but it would be getting it to chill all the tanks then having heaters and the chiller fighting each other sucking heaps of power. I have lots of flow in the tanks and lots of air, it was mainly one room that got really hot. Iam hoping that the air con will run cheaper than heating 10 tanks with separate heaters and will also be able to cool them if needed. Last year I froze a small coke bottle and put that in a 50g that worked but there was plenty of circulation to distribute the temp.