To get the most out of your circulation pump and to make your water flow in the correct direction, point it in the same direction as your filter outlet. Let's assume that your filter is on the right and points left. If you had a circulation pump you would position it on the right side so it also points left (same direction as the filter). Positioning it anywhere else or in some cases the opposite end, will cause negative flow and effectively stop most of the circulation.
Below I illustrate just what I mean. Essentially you want to increase the current around your aquarium so more water is moved and less dead spots remain. The better your water circulation the more fertilisers and CO2 are moved around, the better your plants grow.
Marine guys twigged this a long time ago, and now it's our turn.