What is an overflow box & why you need one

What is an aquarium overflow box?

An aquarium overflow box is a column or box used to control water flow. Overflow boxes - 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.

Why do I need an overflow box?

One of the most important considerations 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. Overflow boxes are designed to channel water from the surface into the box and onward to the filter, hence the name "skimmer box".

Aquariums with overflow boxes are safer.

Another important benefit of properly designed overflow boxes is they 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.

Overflow boxes can augment your existing filtration system.

A great way to boost your aquarium's filtration capacity is by adding bio filtration into the overflow box. Tenecor AquaFoam is an excellent way to do this. Overflow boxes are also great places to add carbon or other water conditioners when they are placed in open mesh bags.

Tenecor AquaFoam placed behind the intake slots will keep smaller livestock from being sucked into the filter. The best AquaFoam for this is our 10 PPI (Pores per Inch). 

What types of overflow boxes 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 you will need to use 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 almost always rectangular, square or trapezoid but can also be oval, round or even elliptical. These 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, which is usually in the cabinet below the tank, can be routed back up through the overflow box.

Drilling the holes.

Drilling holes through glass requires the right tools and is not difficult. The first thing you will need is the proper drill bit ( Actually not technically a "drill bit" but more accurately a "hole saw") The hole saw must be a diamond coated "hollow core" style. These are very common and can be found in almost any hardware store, We offer also offer them as a low cost convenience to our customers.

You will also need an electric drill, duct tape, marking pen, a bottle of water and plumber's putty. Of course, safety glasses are always recommended.

Please note almost all commercially available glass aquariums are not tempered but always check with the manufacturer. Tempered glass cannot be drilled without very specialized equipment. If you suspect your aquarium or some portion of it, is made from tempered glass, do not attempt to drill it. The glass will shatter.