I voted 250 because I know thats what you want to hear. Ive had great success with my 250s but sometimes I wonder if I'd be better off with 400s. I like it blue and 250s arent quite strong enough for an SPS dominated tank if you want to stretch bulb life to the extreme. Im sure others will disagree with me though. And, of course ballast bulb combos will effect par and longevity too.
I really don't want to hear one or the other.. honestly, id rather not spend $500 and just go with the light i have.. but it just seems like a LOT of light. Its actually rare to run into someone who actually RUNS 400w DE bulbs.. or it is around here anyhow.
The MAIN thing is I want a fixture that is all in 1 that I can just sit on top of my tank and take off when needed (like the orbits and the coralifes). If I could FIND one that was 24" that actually HAD 400w in it I would opt for it. I don't want to HANG anything (looks bad with 12' ceilings) and I don't want to have to retrofit a hood.
I'll be running the Outer Orbit 3x 250 watt HQI /T5HO combo on my 180 gallon (also 24" deep). The unit comes with the plastic feet to mount above the tank. I'm pretty sure this is how I'll mount my lights but I've kicked around the idea of mounting it from the ceiling.
I know I'm a newbie at this but 250 watt bulbs for a 24" depth seems more than sufficient from what I've been reading. If I had a really deep tank then I'd go with 400 watt bulbs.
i think you sit the fixture directly on the tank frame. that's why they are about 8-10 inches deep and come with a big glass shield.
in any application i have seen them operating in, this is how the people had it.