You could use a couple of Tunze 6095 or 6105 on the multi controller (or branch them off of the single controller). I think one on each side of the tank would work nicely for you.
Another nice setup would be the smaller 6055, but 4 of them, one in each corner and with a multi controller. That would give you some good flow throughout the tank.
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32"x32"x20" Cube-ish tank
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