I have 2 MP40's on my little old 77 gallon, so my opinion would be 2 more MP40s for your 120 is almost a no brainer. For a while I considered getting an MP10 for the back glass since my 77 is a narrow tank, but the flexibility of the EcoTech drivers allows me to run the 40 on the back glass limited to a lower speed while still acting as a slave of the master, so it follows the selected mode, which in my case is Tidal Swell.
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