Consistenccy is IMPORTANT. You do NOT want to see more than a 2 degree temperature swing in a 24 period. When I ran MH
lights I had a fan on a timer to blow across the tank top when the MH
lights were on. That kept the tank stable within the 2 degree range.
Relying on bags of ice, fozen peas, etc. is fine for when the AC
or chiller doesnt work, but not as a long term solution.
They sell sets (2-6) of computer fans in a clip on enclosure to hang on the back of the tank if you can's place a fan nearby.