I had a 150W MH over a 20g tank on the main floor and it never got above 82 degrees without A/C. I have since switched to T5's so I think it'll be even less of a problem. I used an open top with MH's also so that really helps. I wouldn't rush out to buy a chiller without trying a fan and open top first, or maybe your tank will be just fine as is.
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