I'm running a 24 inch cube, I would say it's largely dictated by your rock work whether you'll need one or two. I had been running off one mp10 for 4 years, and it seems to be adequate, although there were definitely spots that weren't moving as well as others. I move the tank recently and a change in my rock scape came along with it, which resulted in me not being happy with my flow. I just upgraded to the gyre 130. It's expensive, and I'm still playing with it to determine my ideal set up, but I do like it. Mine is dialed way back at the moment, tons of flow. Before I bit the bullet with the gyre I was considering 2x RW4. If you are looking for a little cost option I would go with 2xRW4, but no less than 2... unless you don't have a lot of rock to block the flow. Just my 2 cents
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