How is your current ATO setup?
Also, you don't need a monsterous pump, just one with a high pressure rating...unless you want the water to fill up really fast. Check out something along the lines of Iwaki 40RLT or an Iwaki 20RLZT to get an idea. You can get either of those pumps for a pretty good price from time to time on CanReef.
But, if you want a big pump, I know someone who has an Iwaki 100RLT for sale...
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32"x32"x20" Cube-ish tank
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