What is an aquarium overflow box?

An aquarium overflow is a column or box used to control water flow. Overflows - also known as "pre-filters", "skimmers" and "weirs" are usually attached to the inside back corners of an aquarium. They can also be attached to the back wall, in the center of the tank for unobstructed viewing from all sides, or on one end in peninsula room divider tanks. All overflows have slotted tops so the water can pass but livestock stays behind. They are almost always used in conjunction with an external filter.

Overflow aquariums are healthier and better looking.

An important consideration for a healthy aquarium environment is the tank's water surface area. Much of the dissolved oxygen in the water comes from where air is in contact with the water.

The aquarium water surface is also where much of the undesirable elements build up such as proteins, bio-waste and uneaten food. When these elements build up on the surface, less oxygen will be able to enter the water.

Excess waste on the surface also decomposes faster into the tank rather than being filtered out. Overflows are designed to channel water from the surface into the box and onward to the filter, hence the name "skimmer box".

Aquariums with overflows are safer.

Properly designed overflows prevent total aquarium drainage in case of a power or equipment failure. The water level is allowed to drop only to the bottom of the slots and no further. There is also no chance of a siphon break as with hang on back (HOB) boxes.

What types of overflows are there?

Most overflows are literally boxes or columns. Box style overflows need to have an attached, leak proof bottom as well as sides. They are typically attached to the rear or side walls of the aquarium. With box style "Bird's Nest" overflows, there needs to be a plumbing hole(s) drilled through the wall of the  aquarium or siphon tubes going over the top edge. They are an easy and inexpensive option for retrofits and smaller aquariums but they are not our first "go to" choice. Siphon tubes are not reliable and we do not recommend this type of setup.

Column style overflows extend from the surface all the way down to the bottom of the aquarium. Column style overflows are usually rectangular, square or trapezoid but can also be oval, round or even elliptical. Column style are the preferred type of overflows since they are more versatile.

Another advantage of column style overflows is they can conceal return lines from the filter. Water lines returning to the aquarium from the external filter in the cabinet below the tank, can be routed back up through the overflow box.