Marine and Reef Aquarium Water Chemistry: The Overview

Saltwater chemistry adds salinity and the calcium-alkalinity-magnesium 'big three' to the nitrogen series. Learn the target ranges, how they connect, and why reefs demand tighter stability.

Marine and reef chemistry covers everything freshwater does and then adds several parameters of its own. Seawater is naturally well buffered and mineral-rich, and corals are far less tolerant of swings than most freshwater fish, so a reef keeper tracks more values and holds them tighter. This overview connects the main parameters; the per-parameter guides cover each in depth.

Salinity

Average ocean seawater has a salinity of about 35 grams of dissolved salt per litre, written as 35 ppt or 3.5 percent. In practice reef keepers target a specific gravity of roughly 1.023 to 1.025 for a mixed fish-and-coral tank. The most abundant dissolved ions are sodium and chloride, followed by magnesium, sulfate and calcium. Salinity should be kept steady, as sudden changes stress both fish and invertebrates.

The nitrogen series and nutrients

As in freshwater, ammonia is converted to nitrite and then nitrate, and ammonia and nitrite should read zero in an established system. Reefs differ in their nutrient goal: nitrate and phosphate should be kept low but not zero, because corals need a trickle of nutrients and starve when both are driven to undetectable levels. A commonly cited reef phosphate target is roughly 0.01 to 0.03 ppm, and nitrate is likewise held low rather than absent. See the reef nutrient management guide for the full reasoning.

The big three: calcium, alkalinity, magnesium

Stony corals build calcium-carbonate skeletons, so calcium and alkalinity are central, with magnesium supporting both. Typical reef targets are calcium around 380-450 ppm, alkalinity 8-12 dKH, and magnesium 1200-1400 ppm. Alkalinity here is the same concept as in freshwater, the buffering capacity that resists pH change, and everyone agrees these three must be managed together and kept stable as a set.

pH, temperature and trace elements

Marine pH sits in a narrow band of about 8.1 to 8.4 and is tied to alkalinity and dissolved CO2, just as in freshwater. Temperature is usually held around 75-80°F (24-26°C) with daily swings limited to about 2 to 2.5 degrees. Beyond the big three, corals also use trace elements, which routine water changes normally replenish; see the supplements guide before dosing extras.