are you talking about the vortech pumps? if so, they run on thicknesses of 1/4-3/4" with 1/4-3/8, you'll be using the spacer. and the jbj cubes are 3/8
MP20 would work. I say that because at least you have the option of using it in a larger setup if you go that route in the future. Might have to dial it back a bit. Or go with an MP10 or 2 depending on you r flow requirements.