Larger tanks are build to include internal overflows, this would mean that there are normally two holes drilled in one or maybe two corners of the tank. These two holes are for drain and one for return. The water flows into the overflow and drains down a standpipe to the sump or wet/dry, where it can be filtered by many means, floss, bio balls, skimmer, carbon.. the it is returned via return pump up the other hole,. that would be plumbed and back into the main tank.