Reef Water Chemistry Guide: Every Parameter That
Quick Summary
Reef water chemistry is more complex than freshwater because corals, invertebrates, and coralline algae actively consume minerals from the water to build their skeletons and shells. The parameters that matter most are alkalinity, calcium, magnesium, salinity, nitrate, phosphate, pH, and temperature. Each plays a specific role, and all of them interact.
The single most important principle in reef chemistry is stability. Corals adapt to a range of acceptable values, but they do not tolerate rapid changes within that range. A tank holding steady at 8.0 dKH alkalinity will keep corals healthier than a tank swinging between 7.5 and 9.5 dKH, even though both are within the acceptable range. Every parameter in this guide follows the same rule: consistency matters more than chasing a perfect number.
The Big Three: Alkalinity, Calcium, and Magnesium
These three parameters form the calcification triad. They are consumed by stony corals as they build their calcium carbonate skeletons, and they must be maintained in balance for healthy coral growth.
Alkalinity
Alkalinity measures the water's buffering capacity, specifically the concentration of carbonate and bicarbonate ions. In reef chemistry, alkalinity is the most critical parameter because it is consumed fastest, fluctuates most readily, and has the most direct impact on coral health.
Target range: 7.0 to 11.0 dKH for most reef tanks. SPS corals perform best at 7.5 to 9.0 dKH. LPS and soft coral tanks can run 7.0 to 11.0 dKH comfortably.
Why it matters: Corals use bicarbonate ions from the water to build calcium carbonate skeleton. Every centimeter of coral growth removes alkalinity from the water. In SPS-dominant tanks, alkalinity can drop measurably every day.
Stability requirement: For SPS, daily variation should stay within 0.5 dKH. For LPS and mixed reefs, within 1.0 dKH is acceptable. Swings beyond these thresholds stress corals and can trigger tissue recession or bleaching.
How to maintain it: Two-part dosing (sodium bicarbonate/carbonate solution), calcium reactors, or kalkwasser. Automated dosing pumps delivering small, frequent doses produce the most stable levels. Manual dosing twice daily is the minimum for acceptable stability.
For a deeper exploration, see the full alkalinity guide.
Calcium
Calcium provides the Ca2+ ions that combine with carbonate to form the calcium carbonate skeleton. It is the second most consumed parameter in a reef tank after alkalinity.
Target range: 380 to 450 ppm. Most reef keepers target 420 to 440 ppm.
Why it matters: Without adequate calcium, corals cannot build skeleton regardless of alkalinity levels. Calcium-depleted water produces slow growth and thin, brittle skeletal structures.
Stability requirement: Calcium is less volatile than alkalinity because it exists at higher absolute concentrations (400 ppm vs 8 dKH). A 10 ppm calcium fluctuation is proportionally smaller than a 0.5 dKH alkalinity fluctuation. Keeping calcium within 20 ppm of target day-to-day is achievable with standard dosing methods.
How to maintain it: Two-part dosing (calcium chloride solution), calcium reactors, or kalkwasser. Calcium and alkalinity consumption are linked, so balanced two-part dosing typically keeps both in range simultaneously.
For details, see the calcium guide.
Magnesium
Magnesium stabilizes the relationship between calcium and alkalinity. Without adequate magnesium, calcium carbonate precipitates spontaneously from solution, causing both calcium and alkalinity to fall together despite dosing.
Target range: 1,250 to 1,400 ppm. Most reef keepers target 1,350 ppm.
Why it matters: Low magnesium is one of the most common hidden causes of unstable calcium and alkalinity. If you are dosing alkalinity aggressively and it keeps dropping unexpectedly, magnesium may be the root cause.
Stability requirement: Magnesium is consumed slowly in most reef tanks (primarily by coralline algae). It changes gradually and needs supplementation less frequently than alkalinity or calcium. Testing biweekly is sufficient for most systems.
How to maintain it: Magnesium chloride and magnesium sulfate solutions dosed as needed. Many salt mixes provide adequate magnesium at mixing, so water changes often maintain levels without dedicated dosing in tanks with moderate coral loads.
For details, see the magnesium guide.
How the Three Interact
Alkalinity, calcium, and magnesium exist in a chemical equilibrium. When one is significantly out of range, the others become difficult to maintain.
If magnesium drops below 1,200 ppm, calcium carbonate precipitates from solution. You dose alkalinity and calcium, but both fall because they are combining and dropping out of the water as solid precipitate rather than being consumed by corals. This frustrating cycle continues until magnesium is corrected.
If alkalinity is raised too aggressively while calcium lags, excess carbonate binds with available calcium and precipitates, crashing both. This is why balanced two-part dosing (raising alkalinity and calcium in proportion) is safer than adjusting one parameter independently.
The practical rule: always check magnesium first when alkalinity and calcium are behaving unpredictably. Magnesium is the foundation that allows the other two to stay stable.
Salinity
Salinity measures the total dissolved salt content of the water. In reef keeping, it is expressed as specific gravity (SG) or parts per thousand (ppt).
Target range: 1.024 to 1.026 SG (33 to 35 ppt). Most reef keepers target 1.025 SG (35 ppt), which closely matches natural seawater.
Why it matters: Every organism in a reef tank (corals, fish, invertebrates, bacteria) has evolved for a specific salinity range. Deviations stress osmoregulatory systems, and sudden changes are more damaging than gradual ones.
Stability requirement: Salinity changes of more than 0.002 SG over 24 hours stress corals. The most common cause of salinity instability is evaporation. As water evaporates, salt stays behind and salinity rises. In a typical reef tank, 1 to 2 percent of the water volume evaporates daily, which can shift salinity significantly within a few days if not replenished.
How to maintain it: An auto top-off (ATO) system that replaces evaporated water with fresh RO/DI water is the standard solution. The ATO maintains a constant water level, which keeps salinity stable. Manual top-off works but requires daily attention and introduces more variation.
Measurement: Use a refractometer calibrated with calibration fluid (not tap water). Hydrometers are too imprecise for reef-level accuracy. Calibrate your refractometer every few months.
For more, see the full salinity guide.
Nutrients: Nitrate and Phosphate
Nitrate and phosphate are the products of biological filtration and organic decomposition. They are not waste to be eliminated but nutrients that need to be managed within a target range.
Nitrate
Nitrate (NO3-) is the end product of the nitrogen cycle: ammonia is converted to nitrite, then to nitrate by beneficial bacteria. It accumulates between water changes and is consumed by zooxanthellae, macroalgae, and bacteria.
Target range: 1 to 10 ppm for SPS-dominant tanks. 5 to 20 ppm for LPS and mixed reefs. Soft coral tanks tolerate up to 30 ppm.
Why it matters: Ultra-low nitrate (below 1 ppm) starves coral zooxanthellae, producing pale, fragile corals with poor growth. Elevated nitrate (above 20 ppm for SPS, above 30 ppm for LPS) causes zooxanthellae overpopulation, browning, and reduced skeletal growth.
The ideal is a stable, low-but-measurable nitrate level. A tank holding steady at 5 ppm nitrate grows healthier, more colorful corals than a tank bouncing between 0 and 15 ppm.
How to manage it: Water changes dilute nitrate. Protein skimming exports organic nitrogen before it converts to nitrate. Carbon dosing (vodka, vinegar, or commercial products) promotes bacterial nitrate consumption. Macroalgae in a refugium absorb nitrate biologically. Reduce feeding and fish load to lower input.
For details, see the nitrate guide.
Phosphate
Phosphate (PO4) enters the tank through fish food, fish waste, and some salt mixes. It is consumed by zooxanthellae, algae, and bacteria.
Target range: 0.01 to 0.08 ppm for SPS. 0.02 to 0.12 ppm for LPS. Soft corals tolerate up to 0.20 ppm.
Why it matters: Elevated phosphate directly inhibits calcium carbonate deposition in stony corals, slowing skeletal growth. It also promotes nuisance algae. Ultra-low phosphate (undetectable) starves zooxanthellae and can trigger bleaching when combined with low nitrate.
Phosphate must be managed in proportion to nitrate. The Redfield ratio (approximately 16:1 nitrogen to phosphorus by atoms, roughly 100:1 nitrate to phosphate by ppm in reef terms) provides a useful guideline. A tank with 5 ppm nitrate should have approximately 0.03 to 0.05 ppm phosphate. Major deviations from this ratio stress corals even when individual values look acceptable.
How to manage it: GFO (granular ferric oxide) media in a reactor or media bag adsorbs phosphate from the water. Water changes dilute phosphate. Reducing feeding and cleaning the filter regularly lower phosphate input. Carbon dosing indirectly reduces phosphate by promoting bacterial uptake.
For details, see the phosphate guide.
The Nutrient Balance Principle
Managing nitrate and phosphate together is more important than managing either in isolation. Rapid reduction of one while the other remains elevated creates a nutrient imbalance that stresses corals more than moderate elevation of both.
If your tank runs at 15 ppm nitrate and 0.10 ppm phosphate, lowering both gradually over weeks is safe. Crashing phosphate to 0.01 while nitrate remains at 15 is not. The imbalance disrupts zooxanthellae function and often triggers bleaching or tissue loss.
Always adjust nutrients gradually. Target 10 to 20 percent reduction per week when lowering elevated levels. Test both parameters weekly and adjust export methods to bring them down in parallel.
pH
pH measures how acidic or alkaline the water is on a logarithmic scale. Reef water is naturally alkaline, and maintaining appropriate pH supports coral calcification and overall biological function.
Target range: 7.8 to 8.5. Most reef tanks run between 8.0 and 8.3 during the photoperiod.
Why it matters: Coral calcification is most efficient at higher pH (8.1 to 8.4). Below 7.8, calcification rate drops significantly and corals invest more energy maintaining their internal chemistry. Above 8.5, stress and potential tissue damage occur.
Daily fluctuation: pH naturally rises during the photoperiod (when photosynthesis consumes CO2) and falls at night (when respiration produces CO2). A swing of 0.1 to 0.3 units between day and night is normal. Swings exceeding 0.4 indicate poor gas exchange or excessive CO2 buildup.
Common pH problems: Sealed rooms with poor ventilation allow CO2 from human respiration to accumulate, depressing pH below 7.8 by morning. Opening windows, running a CO2 scrubber on the skimmer intake, or piping fresh air to the skimmer from outside resolves this.
How to maintain it: Stable alkalinity naturally supports stable pH. Kalkwasser (calcium hydroxide) top-off raises pH. A refugium with macroalgae on a reverse lighting schedule (lights on when display lights are off) consumes CO2 at night, reducing the nighttime pH drop. Adequate surface agitation drives CO2 exchange and prevents buildup.
Temperature
Temperature controls the metabolic rate of every organism in the tank. It is not a chemistry parameter in the traditional sense, but it affects how every chemical process in the reef operates.
Target range: 76 to 79°F (24.5 to 26°C). Most reef keepers target 77 to 78°F (25 to 25.5°C).
Stability requirement: Daily variation should stay within 2°F (1°C). Temperature swings beyond this stress corals, and spikes above 82°F (28°C) trigger bleaching.
How to maintain it: A quality heater with a reliable thermostat (or a standalone temperature controller for redundancy). A chiller, fan-based cooling, or room air conditioning for summer. ATO systems (evaporative cooling helps but increases salinity without replenishment). Temperature alarms or smart controllers for early warning.
Testing: What to Test, How Often
Regular testing is the only way to know what is happening in your water. Visual observation of coral behavior provides feedback, but by the time you see symptoms, parameters may have been off for days.
Testing Schedule
Daily (when establishing dosing): Alkalinity. Until your dosing schedule is dialed in and consumption is predictable, test alkalinity every day.
Weekly (routine maintenance): Alkalinity, calcium, nitrate, phosphate. These are the parameters most likely to shift and the ones most directly affecting coral health.
Biweekly: Magnesium, pH (or use a continuous monitor). These change more slowly and need less frequent checking.
Daily (always): Temperature, salinity. Use a reliable thermometer and check the ATO reservoir. These are the parameters most likely to shift suddenly due to equipment failure.
Choosing Test Kits
Alkalinity: Hanna Checker HI772 or Salifert dKH. Both are accurate and easy to use.
Calcium: Salifert or Hanna HI758. Match precision to your needs. For SPS-dominant tanks, higher accuracy matters.
Magnesium: Salifert. Most accurate widely available kit for magnesium.
Nitrate: Salifert or Hanna HI781. Standard aquarium nitrate kits often lack resolution below 5 ppm. Hanna provides better precision at low levels.
Phosphate: Hanna HI736 (ultra-low range). Standard phosphate kits cannot resolve the 0.02 to 0.08 ppm range that matters for SPS. The Hanna Checker is considered essential for SPS keeping.
pH: Continuous pH monitor (preferred) or Hanna Checker HI98107. Continuous monitoring catches overnight pH drops and trending that spot-testing misses.
Salinity: Refractometer with calibration fluid. Do not rely on hydrometers or the salinity reading on ATO controllers.
Quick Reference Parameter Chart
For a printable chart with all values, see the full reef parameter chart.
| Parameter | SPS Target | LPS Target | Soft Coral Target |
|---|---|---|---|
| Alkalinity | 7.5 to 9.0 dKH | 7.0 to 11.0 dKH | 7.0 to 12.0 dKH |
| Calcium | 400 to 450 ppm | 380 to 450 ppm | 350 to 450 ppm |
| Magnesium | 1,300 to 1,400 ppm | 1,250 to 1,400 ppm | 1,200 to 1,400 ppm |
| Salinity | 1.025 to 1.026 SG | 1.024 to 1.026 SG | 1.023 to 1.026 SG |
| Nitrate | 1 to 10 ppm | 2 to 20 ppm | 2 to 30 ppm |
| Phosphate | 0.01 to 0.08 ppm | 0.02 to 0.12 ppm | 0.02 to 0.20 ppm |
| pH | 8.0 to 8.4 | 7.8 to 8.5 | 7.8 to 8.5 |
| Temperature | 76 to 79°F | 76 to 80°F | 75 to 80°F |
Dosing Methods
As your tank matures and coral mass increases, water changes alone cannot replace the alkalinity and calcium consumed daily. Supplementation becomes necessary.
Two-Part Dosing
The most common method. Separate solutions of alkalinity (sodium bicarbonate/carbonate) and calcium (calcium chloride) are dosed in balanced amounts. Products like BRS two-part, ESV B-Ionic, and others provide pre-measured or mixable formulations.
Dose in small, frequent increments throughout the day. Automated dosing pumps delivering every 1 to 2 hours produce the most stable results. Manual dosing twice daily (morning and evening) is acceptable but produces more variation between doses.
Calcium Reactor
A chamber containing calcium carbonate media (aragonite or crushed coral) dissolved by CO2-acidified tank water. The reactor produces a continuous, balanced supply of alkalinity and calcium. Best suited for tanks with heavy SPS loads consuming large amounts of both parameters.
Calcium reactors provide the most stable delivery method but require more setup and tuning than two-part dosing.
Kalkwasser (Limewater)
Saturated calcium hydroxide solution dripped as ATO replacement water. Kalkwasser supplements both calcium and alkalinity while raising pH and precipitating phosphate. It works as a primary method for light to moderate coral loads or as a supplement alongside two-part dosing in heavier systems.
Water Changes as Dosing
Regular water changes with a quality salt mix replenish alkalinity, calcium, magnesium, and trace elements. For tanks with light coral loads (soft corals and a few LPS), weekly 10 to 15 percent water changes may maintain all parameters without dedicated dosing.
As coral mass grows, water changes alone become insufficient. Most reef keepers eventually combine water changes (for trace elements and waste dilution) with two-part dosing or a reactor (for alkalinity and calcium).
Common Myths
"Natural seawater parameters are the goal." Natural seawater alkalinity (approximately 7.0 dKH) is actually at the low end of what most reef tanks maintain. Reef aquariums are closed systems with higher coral density per volume than natural reefs, and slightly elevated alkalinity (8.0 to 9.0 dKH) better supports the calcification demand.
"More water changes means better chemistry." Frequent large water changes can destabilize chemistry if the new salt mix does not match your tank's established parameters. A consistent 10 percent weekly change maintains stability better than erratic large changes. Always match the temperature and salinity of new water to the tank before adding.
"You do not need to dose if you do water changes." This depends on coral load. A tank with a few soft corals and LPS may maintain alkalinity and calcium through water changes alone. A tank with significant SPS coverage will consume alkalinity faster than any reasonable water change schedule can replenish. Testing reveals when dosing becomes necessary.
"Higher alkalinity grows corals faster." Higher alkalinity within the safe range does support slightly faster calcification, but only if the level is stable. Pushing alkalinity to 10 or 11 dKH introduces instability risk (harder to maintain, larger potential swings) that negates the marginal growth benefit. Stable 8.0 to 8.5 dKH is the practical optimum for most tanks.
FAQ
What parameter should I focus on first?
Alkalinity. It is the most consumed, most volatile, and most impactful parameter for stony coral health. Once alkalinity is stable, calcium and magnesium usually follow with basic two-part dosing. Nutrients (nitrate, phosphate) come next.
How often should I do water changes?
Weekly water changes of 10 to 15 percent are the standard recommendation. This dilutes accumulated waste, replenishes trace elements, and maintains overall water quality. Consistency matters more than volume. A reliable weekly 10 percent change outperforms an occasional 30 percent change.
Can I use tap water for my reef tank?
No. Tap water contains chlorine, chloramine, phosphate, silicate, heavy metals, and other contaminants that harm corals and promote algae. Use RO/DI (reverse osmosis/deionization) purified water for both salt mixing and ATO top-off. An RO/DI unit is a fundamental piece of reef equipment.
What salt mix should I use?
Choose a reef-specific salt mix with elevated calcium, alkalinity, and magnesium (Fritz RPM, Red Sea Coral Pro, Tropic Marin Pro Reef, or similar). Standard marine salt mixes for fish-only tanks have lower mineral content. Mix to 1.025 SG and verify alkalinity and calcium match your tank targets before adding.
When should I start testing and dosing?
Begin testing alkalinity and calcium when you add your first stony coral (LPS or SPS). Soft corals alone consume negligible amounts and rarely require dosing beyond what water changes provide. Start dosing when testing reveals consistent declines between water changes.
What is ICP testing and do I need it?
ICP (Inductively Coupled Plasma) analysis tests for dozens of trace elements simultaneously by mailing a water sample to a lab. It provides data on elements that standard test kits cannot measure (strontium, barium, tin, copper, and more). ICP testing every 3 to 6 months helps identify trace element depletion or contamination that standard testing misses. It is valuable but not essential for basic reef keeping.