Try horizontal on the back wall centered with one above the other, one pointing left, one pointing right, I think you will be happy with them but that would be my recommended placement starting point. Be sure to have them at least 8" above the sand, they spin backwards once a day for just about 20 seconds to back flush and they can sweep up sand doing that. The only failures I see are the bearing failing from sand or dry test runs, fortunately that is a very cheap part.