Why spend all that money and then just have to retard the light by having it 4' over the tank or dimming it down(aside from a dusk dawn effect)? I can see the point of having your light power your tank + some emersed plants in the filters ect, then hanging it higher would be necessary (but probably not 4' high. You could probably do the 24" fixture, won't be quite as bright at the ends of the tank, but you would still have plenty of light. Are the finnex Ray II's not dimmable? Seems like you could get the 24" model for much less.