Reef Tank Water Parameters: Alk, Cal, Mag, Salinity and Everything Else

Reef tanks require more water chemistry management than almost any other type of aquarium. The parameters that matter in a freshwater tank — pH and temperature — are just the starting point here. Stony corals consume calcium and alkalinity as they grow, magnesium governs how those elements behave, and nutrient levels (nitrate, phosphate) must stay in a range that is neither too high nor, counterintuitively, too low.

This guide covers every major parameter: what it does, the target range, how to measure it correctly, and what happens when it goes wrong.

Salinity

Target: 1.025–1.026 specific gravity (sg) / 35 ppt

Salinity determines how much dissolved salt is in your water, and it affects osmotic stress on every organism in the tank. Natural seawater is approximately 35 ppt (parts per thousand), and reef tanks should match it closely. The acceptable range for a healthy reef is 1.024–1.026 sg, with 1.025–1.026 being ideal.

Measurement matters here. Cheap swing-arm hydrometers are inaccurate enough to give you a false reading of as much as 0.002 sg — which translates to a meaningful difference in osmotic stress. Use a refractometer calibrated with RO/DI water, or a digital handheld salinity meter (Milwaukee MA887 is a popular budget option). Calibrate regularly.

Evaporation raises salinity over time because only water evaporates, not salt. An auto top-off (ATO) system that automatically replenishes evaporated water with fresh RO/DI is one of the most useful automation investments in the hobby.

Temperature

Target: 76–79°F (24–26°C)

Temperature in this range supports coral metabolism and zooxanthellae function without thermal stress. Many corals tolerate 80–82°F for short periods, but sustained high temperatures trigger bleaching — the coral expelling its zooxanthellae — the same mechanism responsible for mass bleaching events on natural reefs.

The number itself matters less than the stability of that number. A tank that holds 78°F consistently is healthier than one that swings between 75°F at night and 82°F during the afternoon under high light. Invest in a quality heater with an external controller (Inkbird ITC-306A is a popular budget controller), or a heater with built-in redundancy protection.

pH

Target: 8.1–8.3

pH governs the availability of carbonate ions that corals use for calcification. A pH below 8.0 slows coral growth; sustained acidic conditions stress invertebrates. Natural seawater is around 8.2.

pH has a predictable daily swing driven by biology. During the day, photosynthesis by corals and macroalgae consumes CO₂, which raises pH. At night, respiration releases CO₂ back into the water and pH drops. A 0.2–0.3 unit swing between the daily low and high is completely normal. What you are testing when you check pH matters — a sample taken at midnight will look alarming compared to a sample taken at peak light hours.

Running a refugium with a reverse photoperiod (lights on the fuge at night) is one of the best natural ways to raise nighttime pH, because the macroalgae there consumes CO₂ while the display is in darkness. Opening a window near the tank can also help if indoor CO₂ is elevated from breathing, cooking, or a gas stove.

Alkalinity (dKH)

Target: 8–11 dKH

Alkalinity — measured in dKH (degrees of carbonate hardness) or meq/L — is the buffering capacity of the water. It represents the concentration of carbonate and bicarbonate ions available for coral skeletal growth. Every stony coral (LPS and SPS) consumes alkalinity continuously as it builds its calcium carbonate skeleton.

Alkalinity is the single most important parameter to stabilize in a reef tank. A tank running at a steady 8 dKH will grow coral more reliably than a tank that swings between 8 and 12 dKH while chasing the "perfect" number. Rapid drops in alkalinity (more than 0.5 dKH per day) can cause rapid tissue necrosis (RTN) in sensitive SPS corals within hours.

As your coral load grows, daily alkalinity consumption increases. Beginners can get away with weekly two-part additions or kalkwasser in the ATO. High-SPS tanks eventually need a calcium reactor or dedicated dosing pumps to keep up.

Calcium

Target: 400–450 ppm

Calcium is the other half of coral skeleton construction. Coral takes calcium and alkalinity ions from the water in a roughly fixed ratio (approximately 2.5:1 in ppm terms for every unit of dKH consumed). You cannot raise alkalinity without consuming calcium, and you cannot raise calcium without considering its effect on alkalinity balance.

Natural seawater runs around 420 ppm. Most reef tanks are comfortable anywhere in the 380–450 ppm range. Excessively high calcium (above 500 ppm) can precipitate out of solution and bind with alkalinity, crashing both parameters simultaneously — this is sometimes called a "calcium snow" event and it is devastating in a high-SPS tank.

Magnesium

Target: 1,250–1,350 ppm

Magnesium is the often-forgotten third member of the calcification triad, and the one that confounds the most beginners. Magnesium keeps calcium and alkalinity in solution. When magnesium is depleted (below 1,200 ppm), calcium and alkalinity precipitate out of the water column even when you are dosing them aggressively. You will watch your numbers drop every day no matter how much you add, and the problem is not your dosing — it is your magnesium.

Test magnesium at least monthly. Correction is easy with magnesium supplements (magnesium sulfate or chloride), but raise it slowly — no more than 50–100 ppm per day.

Nitrate

Target: below 20 ppm for LPS/softies, below 10 ppm for SPS

Nitrate is the end product of the nitrogen cycle — far less toxic than ammonia or nitrite, but still damaging at elevated levels. High nitrate (above 40 ppm) turns coral tissue brown as zooxanthellae over-proliferate under the nutrient load, and suppresses growth.

However, ultra-low nitrate is also a problem. Many hobbyists chase zero nitrate and end up with bleached, pale corals that lack the nutrients their zooxanthellae need. A small amount of nitrate — 1–10 ppm for SPS tanks — is healthy. The goal is not zero; it is stable and low.

Phosphate

Target: below 0.1 ppm for SPS, below 0.25 ppm for LPS/softies

Phosphate at high levels inhibits coral calcification and causes nuisance algae blooms. GFO (granular ferric oxide) reactors and phosphate-absorbing media are common tools for controlling it.

Like nitrate, too-low phosphate can trigger bleaching. The Redfield ratio — the natural relationship between nitrate and phosphate in ocean water — is roughly 16:1. A tank with 5 ppm nitrate and near-zero phosphate is out of balance, and corals will show stress. Maintain nitrate and phosphate in proportion, not just individually low.

Testing frequency and equipment

For a new tank in the first three months, test alkalinity every 2–3 days, calcium and magnesium weekly, and nitrate/phosphate weekly. A mature, stable tank with light coral load can get by with weekly alkalinity and bi-weekly everything else.

Invest in quality test kits. Salifert is the hobbyist standard for alk, cal, and mag. Hanna Instruments checkers (HI755 for alkalinity, HI713 for phosphate) give digital readouts and remove color-matching subjectivity. ICP-OES water testing (mail-in lab testing from companies like Triton or ATI) gives a complete elemental panel quarterly and is excellent for diagnosing mysterious tank problems.