water changes

Water changes are a staple of good saltwater maintenance. Larger (approximately 200 US gallons (760 L)) aquariums are much more stable and water changes may not need to take place if the nitrogen cycle has fully established itself in the tank, although this is a controversial statement among aquarists. Water changes are used to maintain balance of calcium, carbonate alkalinity, and magnesium which are rapidly depleted in a reef aquarium, while also maintaining levels of other trace elements as well as removing toxic solutes which may accumulate from many different sources and cannot be removed by even advanced filtration methods. Supplements are needed (such as calcium) when regular water changes alone are not able to maintain adequate levels, particularly those of calcium, carbonate, and magnesium. Water changes involve removing a fraction of the total volume of the aquarium, replacing that water with new pre-mixed saltwater. Pre-mixed saltwater has been dechlorinated and/or dechloraminated—typically with an additive such as bisulfite or through filtering. Water should be brought to the same temperature if more than a 5% change is occurring. Salinity should match that of the aquarium, or be dosed very slowly if altering the salinity. Aging and aerating saltwater (such as in a bucket with a powerhead or airstone) is recommended as good practice to allow the pH to stabilize.

Replacement water should be of the same source as the aquarium, whether it be reverse osmosis (RO), de-ionized (DI), distilled or from a municipal supply, in order to avoid drastic changes in water chemistry. In cases where one is replacing a tap water-based salt mix with a reverse osmosis-based salt mix, the replacement water should be added slowly over the course of several hours to avoid sending the aquarium inhabitants into osmotic shock. However, large water changes are not advised under routine circumstances anyway, so this is really irrelevant. Municipal, or tap water, is not recommended for a marine aquarium as it often contains high levels of nitrates, phosphates, and silicates and other dissolved solids which fuel the growth of nuisance algaes, particularly diatoms, which appears as a rust colored powdery algae and grows in the overabundance of silicates present in all tap water. Water filtered by a four stage process including mechanical, carbon, reverse-osmosis, and de-ionizing components is recommended as this can provide the easiest route to absolutely pure water. .