[mike31154]

In any newly set up system with fresh sand and an MP40, you'll likely have some movement for a while, until the sand 'matures'. Aside from location, the mode and how high you crank up the power will have a great impact on what happens to your substrate. Once the bacteria, critters, minerals in the water start colonizing & encrusting the sand grains, you should be fine. My 77 is the standard height and I have two MP40s about 3-4 inches from the surface, no issues with sand unless I or one of the critters disturbs it. No idea what kind of substrate/sand I have since I purchased everything used. Seems to be variety of small/medium/large stuff. You can always keep the pump on a relatively low power setting until you get the landscape sorted out.