90 gal cube

I'd say two MP-10s. I just got my first pair of 40s and they're a lot on my 300 gallon tank. I've had an MP-10 before and a couple should do nicely :)
 
I'm a big fan of syncing two units for flow from more than one area. I guess you could try one MP-40 or two for that matter. It would just have to be turned down quite a bit because they are very powerful.

Check out ecotech's flow calculator on their site. That might give you a better idea of the plausibility of either.
 
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