Before investing in a chiller I would look at fans. A cheap 7" WalMart $8 clip on fan lowers my 100G 5-7 degrees all by itself. I had to put my two 4" computer canopy fans on a timer with the MH lights and the clip on over the sump on a Ranco controller (another timer would have worked but I had the controller) or the tank got too cool. Thats in Phoenix and we have our home AC on a setback thermostat so its warm in the house during the day when we are at work and does not cool down until late afternoon before we come home.
Fans work wonders. My 1/4HP chiller is now only a back up and has not run since April 2008 !