For moving the sand without clouding: How about draining the water from the 55 (into buckets or a big tub or whatever), put the rock into the 75, scoop the sand out of the 55 and put it in the 75, and THEN fill the 75 with old/new water. If you pour the water onto a plate or shallow bowl in the tank, it won't stir up the sand.
This way you're not trying to move sand through water, and it won't get cloudy.