That's why I was curious what kind of coloration changes you were wanting to see. If you are just looking to do a simple dusk/dawn effect for aesthetics, then you could get away with adding a few T5 lamps. I'm not sure how many lamps it would take. 4? 6? 8? I guess it depends on how intense you want the "actinic" portion to be and if you're wanting a slightly blue color (assuming they'll be Blue+ and run w/ the MHs).
The way I approached it was that the tank is pretty comparable to four 150g tanks stacked next to each other. A standard 120g would have six overdriven T5s (or maybe 8 normally driven), so maybe do something like 7 overdriven for the extra depth on the 150g (or 9 normally driven?). I figure you'd benefit from some of the light spreading onto the other "quadrants" since they're all next to each other, but at the same time that little extra may be needed for the additional depth over 24". That would mean roughly 28 overdriven lamps (2240w) or maybe 36 normally driven lamps like originally mentioned (1944w). It did seem odd to have such a big difference in wattage, especially with so many usual benefits from the T5s. However, the "normal" MH recommendation (one unit per 2'x2' and 400w on >24" depth) would mean eight MHs on that tank and 3200w! (or maybe the 1000w like Jason said). Comparing the "normal" MH vs T5 setup would show way more benefit for the T5s, but if three 400 watters is doing the job then it's pretty hard to beat. The heat transfer and increased tank temp w/ halides is a good point though. Wonder how much it would change the temp on that setup.
I read it as only having 6 bulbs TOTAL. 6 six bulb fixtures would work well. Man, I'm such a moron sometimes.:rolleye1:
