go with the next size under the AC 500 and run a AC mini inpeler. that way it will lower the flow rate and you cane have a fuge and a place to put you heater.
Here are some links I found that might help
http://www.nano-reef.com/forums/show...threadid=51500
http://www.nano-reef.com/forums/show...threadid=51361
http://www.nano-reef.com/forums/show...threadid=51356 the last one might not help too much but it will give you an Idea.
Brad