1. The problem statement, all variables and given/known data A tank at sea level with fresh water measuring 36"x19"x30" contains a one inch bed of sand. The sand particles are .21 mm in size and have a specific density of 2.6. The water in the tank recirculates at a rate of 1,000 gph. There is a one inch return at the surface of the tank, and there are three two inch drains drilled into the base of the tank. The question is, what is the minimum height of the drain above the surface of the sand such that the sand will not be sucked down the drains? 2. Relevant equations The water flow rate provides the force. The particle size and specific gravity provide the mass. We want acceleration of the sand particles to remain at zero. The equation should be some variation of F=ma, but the actual equation is not provided. 3. The attempt at a solution This is an actual situation I face with an aquarium that I can neither solve myself nor obtain assistance from anyone I know. I place the question here, because it most closely approximates a home work question. I am looking for assistance in obtaining possible solutions or at least an equation that expresses the problem.