Reef Tank Water Chemistry for Beginners: Salinity, Alkalinity, Calcium & More

Once your tank is cycled, water chemistry is what keeps it stable long-term. None of these numbers matter in isolation — what matters is consistency. Here’s what each parameter actually does, the ranges we target, and when to test for it.

• Salinity
• The Purpose of an ATO
• Alkalinity
• Calcium
• Magnesium
• Trace & Macro Elements
• Nitrate & Phosphate: Ideal Ranges
• Testing Schedule & Equipment

Salinity

Salinity is the foundation everything else is built on. Reef tanks should be mixed and maintained at a specific gravity of 1.024–1.026 (about 35 ppt), matching natural seawater. Use a refractometer rather than a hydrometer for accuracy, and calibrate it every couple of months with a proper calibration fluid — never plain RO/DI water, which will throw off the reading. Swings in salinity stress corals and fish far more than a stable reading at the “wrong” number, so consistency matters more than chasing an exact figure.

The Purpose of an ATO

An auto top-off system (ATO) automatically replaces water lost to evaporation with fresh RO/DI water. Here’s why that matters: when water evaporates, only the H2O leaves the tank — all the dissolved salt and minerals stay behind. Without a way to replace that lost water, salinity creeps upward day after day. An ATO uses a water level sensor and a small pump to keep the water level (and therefore salinity) constant, and it has the side benefit of protecting your return pump from running dry. It’s one of the simplest pieces of equipment to add and one of the most effective at removing daily maintenance and salinity swings from the equation. For a full breakdown of reservoir sizing and our brand picks, see our dedicated guide to ATO systems.

Alkalinity

Alkalinity measures the water’s capacity to buffer pH and, more importantly for reef tanks, it’s the building block corals use (alongside calcium) to grow their skeletons. Most reef tanks target 8–12 dKH, though many hobbyists — especially beginners — keep it closer to 7–9 dKH for an easier margin of error. Alkalinity is typically the first parameter to drift in an established reef tank as coral growth consumes it, so it’s worth testing more frequently than calcium or magnesium once your tank matures.

Calcium

Calcium is the other half of what corals use to build their skeletons. The target range for most reef tanks is 380–450 ppm. Calcium and alkalinity are linked to each other — if one is off, it affects how stable the other can be — so they’re usually dosed and tested together.

Magnesium

Magnesium doesn’t get used up nearly as fast as calcium or alkalinity, but it plays a critical supporting role: it keeps calcium and alkalinity from precipitating out of solution when both are elevated. The target range is roughly 1300–1350 ppm (a broader acceptable range of 1200–1400 ppm is fine). If you’re struggling to keep calcium and alkalinity stable no matter how carefully you dose, low magnesium is often the hidden cause.

Trace & Macro Elements

Calcium, alkalinity, and magnesium are the major elements — the ones corals consume in the largest quantities and the ones you’ll dose and test most often. Trace and minor elements (iodine, iron, strontium, and a long list of others) are used in much smaller amounts, but they still matter for coral color and health. In a new tank, weekly water changes are usually enough to keep trace elements replenished for the first several months. As your tank matures and coral biomass increases, a dedicated trace element supplement (or an all-in-one dosing solution) becomes more worth adding to the routine.

Nitrate & Phosphate: Ideal Ranges

This is the parameter beginners get wrong most often: zero is not the goal. Truly zero nitrate and phosphate starves corals, causes pale or bleached-looking tissue, and often triggers dinoflagellate or cyanobacteria outbreaks — the exact problem you’re trying to avoid. Modern reef keeping targets low but detectable levels: nitrate around 3–15 ppm and phosphate around 0.03–0.10 ppm. For a newer tank (under 18 months) or an SPS-dominant system, aim for the lower end of that range — nitrate under 5 ppm and phosphate under 0.05 ppm. The goal is stability within a healthy range, not chasing zero on a test kit.

Testing Schedule & Equipment

We use and recommend Hanna Instruments checkers at the shop — their digital colorimeters take the guesswork and color-matching error out of testing and give you a precise, repeatable number instead of eyeballing a color card. For a new tank, test ammonia and nitrite every couple of days until you’ve confirmed the tank is cycled. Once established, test alkalinity, calcium, and magnesium once or twice a week, and check nitrate and phosphate weekly. Salinity should be checked with every water change at minimum. Testing consistently — and writing down what you find — is what lets you catch a slow drift before it becomes a problem your corals show you the hard way.

Not sure which test kits you need or how to read your results? Stop in and we’ll walk through it with you. If you haven’t already, check out our guide on how to cycle a new reef tank, our guides on UV sterilization and protein skimmers, and our Coral Care Guide for species-specific requirements once your parameters are dialed in.

Stability Beats Perfect Numbers

Corals adapt to a fairly wide range of calcium, alkalinity, and magnesium levels over time — what actually causes stress and tissue damage is a sudden swing, not a parameter that’s slightly outside the “ideal” range but stable. If your tank has been running steady at a slightly lower alkalinity for months with healthy coral growth, resist the urge to chase the textbook number quickly; make any correction gradually, over a week or more, rather than in a single large dose.

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