Aquarium Overflows allow water to travel from your display tank down to a sump or other filtration devices. The overflow will set the water level in your display tank, and any water that makes it over the edge of the overflow box will then fall down to your filtration system. Their are multiple styles of overflows and we HIGHLY recommend a traditional internal overflow that requires a drilled aquarium. However, there are HOB (hang on back) styles of overflows that utilize a siphon principal to bring the water up and over the edge of the aquarium before it can travel down to the sump which does not require any holes in your aquarium. HOB overflows do have some inherent risks for flooding which is why we recommend a internal overflow.