Are you talking about your overflow from your display tank to your sump/'fuge? If so, you can either get a tank drilled for overflows, or get a HOB overflow (often called an overflow box) for a tank that has not been drilled by the manufacturer for overflows.
When you use a HOB overflow or an overflow kit from a manufacturer for a tank, they are rated at a certain GPH of flow, you will want to get a return pump that has a similar GPH rating, so that it can handle the water drained by the overflows. For example, if you have an overflow box rated at 300 GPH, you're going to want a return pump that is capable of 300 GPH, dont forget to add in head loss, so you'd want that 300 GPH at ~3 feet of head, not at 0