That depends on where the tank is going to sit and how you plan to set it up. For instance if you plan on doing a closed loop a hole right in the bottom center is a good spot. If you want to to install an overflow box then put the hole in accordance to where you want your overflow. If you dont want an overflow box than drilling the hole on one of the vertical panes along the top third of the tank is good. (like how you would drill an emergency overflow)Where exactly is the best place to drill a hole for an overflow?????