You should put it on the top. That way you're taking off the top layer only, which gets the film buildup. Also, if something goes wrong with your pump, your whole tank isn't going to drain down. The point is that your sump pumps back into the main tank, and only the volume of water that is pumped up is displaced back down to the sump. If you put the hole at the bottom, it will drain the whole tank and if the sump pump can't keep up, you're screwed.