you have a 24" deep tank. in my humble opinion 400's are a waist of power and heat in this depth and you can get just as much if not more bang for your buck running 250 watt SE bulbs on tar HQI ballasts.
another thing to conceder is there is no such thin g as a 400 watt HQI ballast, it is a son Agro ballast used for running mercury vapor bulbs that happen to run 400 watt MH bulbs but they over drive the bulb by about 30 watts causing a shorter life span. a real 250 watt HQI ballast does not over drive the 250 watt bulb but rather runs it to proper design specs for the European bulb design which results in a longer bulb life from my experience..
So my recommendation would be 3 HQI 250 watt ballasts running something like AB 10000K, or SA 10000K bulbs as these two have higher outputs than the other ones I have seen. the AB is actually a true 13K bulbs and it has an ice blue color and the SA are a 10000K bulb with a tint of green or yellow that you can't see if you are running actinic.
Steve
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 *everything said above is just my opinion, and may or may not reflect the views of this BBS, its Operators, and its Members. If cornered on any “opinion” I post I will totally deny having ever said this in a Court of Law…Unless I am the right one*
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