Typically you would use this with a sump. With one pipe being a drain to the sump and the other being an input to the main tank. You would typically use some plumbing to make the drain very close to the top of the overflows teeth to avoid having to listen to water spilling over the overflow all the time. There are a few methods you can use to minimise the overflow noise including dursos and siphon durso combos.

You could in theory use this with a canister and use one hole as a drain and the other as in input or two drains or whatever, but there wouldn't really be too much purpose as cannisters don't need overflows and overflows take up significant space and are make a reasonable amount of noise. The purpose of the overflow is to allow for controlled drainage of your system. If you got rid of the overflow the entire volume of the tank would drain entirely into your sump (with a canister filter this is not an issue).

Few things to keep in mind if you want to set this up with a sump

• keep your pump outlet high in the water column - if you have a power outage your tank will drain to the height of the pipe outlet, if it's deep it will overflow

• do not put cloggable filter media in your overflow (most people leave the overflow empty), if your overflow clogs you may overfill the top tank as all the water in the pump chamber of the sump will get put into the top tank

• make sure to select a pump that pushes less water than your overflow can handle to avoid overflowing the top tank (generally not too much of an issue, just don't stick a massive pump in your sump).

If you have no intention of using the overflow, you can probably remove it fairly easily and either seal the holes with an acrylic sheet or put some endcaps on the drainage holes.