I have oak canopy that was 9" originally. I'm using it for my reef tank. I've gone with a retro fit from hellolights.com with (2) 150watt MHs and (2) 55watt T5's. I built up my canopy about 8", because once you put the MH fixture inside you might only have 3" left of room from the water to MH. This way I have 9" of space between the MH and water. People do recommend up to 12" of space in order not to run into heating issues.
heres some retro fits from them. http://www.hellolights.com/index.asp...S&Category=558 http://www.hellolights.com/index.asp...S&Category=707