Like MyReef, I have CSL 1/3HP, noisy, generates a lot of heat, I couldn't live without it (at least my tank couldn't) it allows me to run the tank at +/- 0.5 deg 24hrs per day. Mine is also in the garage, but this is a serious consideration, they are a lot noisier than pumps and generate a lot of excess heat so consider where it is going. It's hard to say what size you need - it depends on a lot of environmental factors, but mine keeps around 300 gallons of water at 79 deg when the day and the room gets up to 100 - there is a time factor and other considerations here, but my guess is a 10 degree drop over a few hours.