A Refugium is an appendage to a marine, brackish, or freshwater fish tank that shares the same water supply. For some applications water flow is limited in order to protect plants or animals that need very slow water. The refugium light cycle can be operated opposite to the main tank, in order to keep the total system pH more stable (due to the uptake of acid-forming CO2 by the refugium during its daylight hours). One size guideline is 1/10th main tank volume. A refugium may be used for one or more purposes such as denitrification, nutrient export, plankton production, circulation, surface agitation, oxygenation or aesthetic purposes.