Refugium & Sump Design Guide — The Key to Stable Marine Water Quality

Powerful allies for long-term water quality stability in saltwater aquariums are the sump (filtration tank) and refugium (a supplementary tank where macroalgae is cultivated). This system, installed beneath the main tank, provides numerous benefits including increased total water volume, centralized equipment, and enhanced biological purification capacity. This article covers sump and refugium design through operation in detail.

Basic Concepts of Sumps and Refugiums

A sump is a supplementary tank installed below the main aquarium, connected via an overflow system. There are three major benefits of using a sump. First, the increased total water volume makes sudden water quality changes less likely. For example, if the main tank holds 200L and the sump 100L, the total system volume is 300L. Second, equipment like protein skimmers, heaters, and auto-top-off devices can be consolidated in the sump, keeping the main tank looking clean. Third, using part of the sump as a refugium enables biological water purification. A refugium (meaning "refuge") is an area where macroalgae and microfauna (small crustaceans and microorganisms) are cultivated.

Sump Baffle Design

The sump interior is typically divided into multiple sections using baffle plates. Here is a basic three-chamber configuration. The first chamber (inflow side) houses the protein skimmer. Water falling from the main tank enters this area first, where organic matter is processed by the skimmer. Installing a filter sock to physically catch large debris is also effective. The second chamber is the refugium area where macroalgae is grown. Adjust baffle height so the water level in the second chamber remains high. The third chamber is the pump chamber, housing the return pump, heater, and auto-top-off float switch. Baffles can be made from acrylic or glass bonded with silicone and are suitable for DIY construction.

Choosing and Managing Refugium Macroalgae

The standard macroalgae for refugiums is Chaetomorpha (commonly called "chaeto"). This green filamentous algae has excellent nitrate and phosphate absorption capacity and is easy to manage. It grows with light, and maintenance simply involves thinning when it overgrows, requiring no special care. Caulerpa is also popular but carries the risk of "crashing" -- dissolving all at once during sexual reproduction and rapidly degrading water quality. This requires 24-hour lighting to suppress reproduction. For beginners, Chaetomorpha is by far the better choice. Running refugium lighting on the opposite cycle from the main tank (refugium on while main tank is off) helps suppress pH fluctuation. As the algae absorbs CO2 through photosynthesis and raises pH, it compensates for the nighttime pH drop.