what kind of lighting are you running and how many degrees are you wanting to pull the tank down to. those are all factors into sizing a chiller for your tank. Also what the average temp swing from morning till the lights go out after the light cycle if your only seeing a small amount temp change you may just think about doing a few fans to cool it down.
1/3 hp is minimum I would recommend. 1/4 hp will be fighting to keep your tank chilled. A chiller is an ac for your tank. If you have an under powered ac for a house it will just stay on or be constantly turning on to keep the house cooled.
I am running 3- 250 metal haylides, I have a 4 degree temperature swing from the morning till the lights go off at night. The chiller is a cl-650, I looked it up and it said 10 degrees cooling up to 180 gallon.
