I have one over a 24x24x8 cube, 12" above the water. I only run the lights 2-3 hours though. Right around that time the water temp reaches 81, from 79.
I'm going to have to think of a way to elevate the canopy, like building it up from underneath with more wood so I can get some more height. Do you ever get close to Westminster/Thornton/Broomfield?
Wired the plug to the socket, hopefully this will make it more DIY friendly. The reflector is slightly bent on the edges, pics to come when I get home....