An overflow or prefilter box setup allows the tank water to be cleaned of excess debris, particles and other unwanted organics (dissolved organic compounds) before it passes through the bio-media in the wet/dry chamber. By using this process it helps to prevent the bio-material from getting clogged and dirty, thus contributing to the build up of nitrates in your aquarium. A built-in overflow is the most efficient method of draining aquarium water to a filter. If your aquarium does not have built-in overflow, Overflows or Prefilters are the best means for getting water from an aquarium to a filter system without expensive modifications to the tank.