What is an aquarium sump?
An aquarium sump is at its simplest, a water vessel usually smaller than the main display aquarium it is attached to. Sumps can be plumbed in a variety of configurations. The most common method is direct plumbing to the aquarium by way of an overflow box or hang on the back (HOB) skimmer. In this setup, the water will flow by gravity to the sump. The filtered water is returned back to the aquarium by means of a pump. Sump pumps can be submersible and sit inside the sump itself or external.
Sumps are extremely customizable. Heaters, protein skimmers, and other equipment can be placed inside the sump instead of cluttering up the main tank.
Sumps are suitable for both fresh and saltwater.
Saltwater sumps are are a proven way to filtration method and are widely adopted. Freshwater applications are simpler since much of the equipment in saltwater setups is not necessary. Freshwater sumps also improve the health and stability of the aquarium since more water in circulation is greater.
The advantages of sump filtration.
Sumps have several advantages compared to power and canister filters. As mentioned above, the biggest advantage is the increase in water volume in circulation. Accidental overfeeding or a dead fish will not cause waste levels to rise as high. Larger water volumes are also more thermally stable. Brief power outages will not be as fatal. Water chemistries also will be more stable because of the greater volume.
Sizing your sump.
There is no set rule for the correct sump size. Sump sizes depend on what are the important objectives. Simple sumps with biological filtration and a pump are enough for many setups. Larger sumps with multiple chambers can be used for running additional equipment such as protein skimmers, reactors and UV sterilizers.
Sump capacity also needs to be large enough to handle the volume of water that will drain to the sump in the event of a power outage. This includes the water in the plumbing as well as the any water that would continue to drain from the tank itself.