da colts":1wgj9u6c said:i have a 1/4 horse on my 125 and it keeps the temp at 76 or 77, for your size tank a 1/3 or 1/2 will probably work the price differece between a 1/3 and a 1/2 is not that much and a bigger chiller will mean that temp will stay constant without the chiller burning itself out by coming on all the time.