I had a vacuum system I counldn't even get sand to upflow because the tube diameter was too large on the uptake and the drain pipe is a little small - simple solution for a gravel system: either icrease intake tube size or (easier) restrict outflow tube; better still, I am not draining a vast amount of tank water when I vacuum the substrate and stirring is easier. That said, I added a smaller intake tube just to pull sand up nicely (goes 1/2 to 3/4, allowing sirt to be removed but the sand falls back into place - nice.)