Mark,
You might want to read Anthony Calfo's book, Coral Propagation. It contains a lot of good info on how to set up an effective green house. Calfo makes the point that its easier, and more cost effective, to heat a greehouse up north than it is to cool one in the South. Of course, that is in regards to a large coral farm like his in PA.
I know a guy in New Orleans that has run a very efficient coral farm for several years. He uses a geothermal system for cooling. His coral farm led him to open the best LFS I have ever seen. Most of the product from the coral farm goes into his LFS.
In setting up your geothermal system remember that plastics, like PVC, don't have very good heat exchange properties, as compared to metals such as copper or titanium. The way to overcome this lack of heat exchange is to oversize the length of the cooling loop. More surface area means more cooling. Of course the longer the loop, the larger the pump you will need to drive it because of losses due to drag.
I just thought I would throw out a few things for you to consider in designing your system. Based on Tim's experiences, it seems that you won't need to do a huge amount of cooling.
Gary
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano