Overflows are a method of taking water out of a tank and to a sump. Two advantages of a weir are the redistribution of water with the highest oxygen saturation and the filtration of water with the highest possible dissolved organic nutrient concentration. Overflows draw aquarium surface water and since the surface is one of the main gas exchange locations water with the highest oxygen content is circulated through the sump and redistributed throughout the aquarium. Due to the same principle that protein skimmers take advantage of, the air/water interface at the surface of an aquarium attracts the highest concentration of dissolved organics which is a highly desirable feature since these are the principle nutrients we attempt to remove from the aquarium whilst the water passes through the sump.
It is important to note that it is not possible to set up a system of two pumps, where water is pumped from one tank to another using a one pump, then from the second back to the first tank using another pump. It is impossible to balance the flow rates of the pumps so that they exactly match. So in all cases will at some point in time end up with water on the floor.
Located either within the tank or attached to the side of the tank. The most common and preferred method of getting water from the tank. Weirs and Overflow boxes can be fabricated in many styles, shapes and forms and can be disguised if made with tinted glass or overlayed with black acrylic panels.
Commonly an weir/overflow box is vertical, can be situated centrally on the back glass panel or side glass panel in the rear corner or both rear corners (in the case of a dual overflow design on larger aquariums) or diagonally across the rear corner(s)with a comb, eggcrate or Gutter Guard to prevent fish going over the side and ending up in the sump. On some Peninsular orientated aquariums (situated end on to the wall) the entire side/end of the aquarium is utilised as the overflow box with a full width comb or guard thereby concealing the overflow box as the end of the aquarium.
Becoming more increasingly popular is the horizontal weir/overflow system which can be full width across the back of the aquarium (Calfo or Coast to Coast Overflow)or partially across the back of the aquarium and with a total depth of around 150mm (6"), benefits of this style of weir is greater transfer of surface water removal and being less unsightly than a vertical weir/overflow fabrication. Additionally, combs or mesh guards can be elimated because of the shallow depth and protection of fish going down to the sump using correctly positioned pipes with strainers or fabricated elbows.(see the resources link below to Silent & Fail-Safe Aquarium Overflow System).
An alternative style weir/overflow is to silicone an external box to the aquarium and either cut out a section of glass in the rear or side panel to act as the overflow or drill several holes to allow surface water to flow into the external box. Note, there have been cases where the rear panel has cracked using this method.
These are an overflow that hangs on the side of the tank, which does not have to be drilled. The siphon box is essentially two boxes, one inside the tank and one outside that are connected by a siphon. Care must be taken to ensure bubbles do not get trapped in the U tube that connects both sides, otherwise the siphon will break. These can have problems associated with the siphon not restarting after a power failure. Some designs include a suction line on the top of the siphon that can be connected to a small pump to ensure the siphon is always maintained.