I have a 75g as well and I have the MJ mods, however they are not on MJ's. I put the props in my Koralia 4's. Not quite as laminar and someone ran a bag test on one and it is closer to 3000gph and a lot quieter. I have two of them along with another regular Koralia 4 (just need to buy the kit for it)and a Koralia 2 in my tank. So if the numbers are right, I have about 8000gph in my tank which is over 100x turnover and when the third K4 is done, it will be close to 10,000gph (if you believe the numbers they publish). But to answer original question, two will be fine but it will move some of the sand around unless you keep them pointed level or upward. I have a bare bottom tank so it doesn't effect me.