either on the bottom(vert) or sideways(horiz). I would do sideways though. put the inlet on the bottom this way you force bubbles up. the reason I wouldnt put the posts on the bottom is the pump must work harder to "push" the water up, let gravity pull it down and then 'Push it up again. if you mount it sideways you kill a lot of the push. water flows on a horizontal line with little pressure.and the push /pull is now limited to the bends in the tube rather than the straight parts.
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