If you have the money: buy a very smaller in-line chiller. From what the manufacturers tell me a 1/15hp chiller running at >100GPH will drop a 40 gallon tank at least 7-8 degrees (I'll verify that when I actually get it all set up). The upside: it's perfectly stable, essentially automatic and nearly maintenance free, and it's able to keep your tank down below 65 degrees. The downside: the 1/15hp ones seem to run over $200 and it gets worse if you have a huge tank. Anyway, if you don't have the money agreed with the earlier post, check out the article on cooling.