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Reef Tank Parameter Chart: Complete Water Chemistry

Reef Tank Parameter Chart: Complete Water Chemistry

Quick Summary

Reef water chemistry involves a dozen or more measurable parameters, and keeping track of ideal ranges can be overwhelming. This chart consolidates every important reef parameter into a single reference. The target values here reflect natural seawater levels and established reef keeping best practices. For most reef tanks, stability within these ranges matters more than hitting exact numbers. Test regularly, make changes gradually, and focus on the parameters that your specific coral mix depends on most.

Master Reef Parameter Chart

This table covers the core parameters that every reef keeper should monitor. Ranges are given for standard reef tanks (mixed reef with SPS, LPS, and soft corals). Adjustments for specific coral types are noted in the sections below.

Parameter Ideal Range Natural Seawater Test Frequency Priority
Salinity 34 to 36 ppt 35 ppt 2x per week Critical
Specific Gravity (at 77°F) 1.024 to 1.026 1.0264 2x per week Critical
Temperature 76 to 80°F (24 to 27°C) Varies Daily (controller) Critical
pH 7.8 to 8.4 8.1 to 8.3 Weekly Important
Calcium (Ca) 380 to 450 ppm 420 ppm Weekly Critical
Alkalinity (dKH) 7.0 to 10.0 dKH 7.0 to 8.0 dKH Weekly Critical
Magnesium (Mg) 1250 to 1350 ppm 1280 ppm Monthly Important
Nitrate (NO₃) 1 to 10 ppm ~0.5 ppm Weekly Important
Phosphate (PO₄) 0.01 to 0.10 ppm ~0.005 ppm Weekly Important
Ammonia (NH₃/NH₄⁺) 0 ppm 0 ppm As needed Critical (if detected)
Nitrite (NO₂) 0 ppm 0 ppm As needed Critical (if detected)
Potassium (K) 380 to 420 ppm 399 ppm Monthly Moderate
Strontium (Sr) 8 to 12 ppm 8 ppm Quarterly (ICP) Low
Iodine (I) 0.04 to 0.08 ppm 0.06 ppm Quarterly (ICP) Low

Parameter-by-Parameter Breakdown

Salinity

Salinity is the most foundational parameter in a reef tank. It determines the concentration of every dissolved ion and directly affects osmoregulation in every organism. A stable 35 ppt matches natural seawater and provides optimal conditions for corals, fish, and invertebrates.

Measure with a calibrated refractometer. Swing-arm hydrometers lack the accuracy needed for reef tanks. An auto top-off system is the most effective tool for preventing salinity drift from evaporation. For a complete guide, see Salinity Explained.

Temperature

Most reef organisms thrive between 76 and 80°F. Temperature stability matters as much as the target range. Swings of more than 2°F within a 24-hour period can stress corals and trigger immune suppression in fish.

Use a digital thermometer (not a stick-on strip) for accurate readings. A heater controller with a probe and a chiller or fan for warm months provide the best stability. In most tanks, a quality heater rated for the tank volume and placed in a high-flow area maintains temperature within 1°F.

pH

Reef pH naturally fluctuates throughout the day. It drops at night as respiration produces CO₂ and rises during the day as photosynthesis consumes CO₂. A range of 7.8 to 8.4 is normal. Consistent readings below 7.8 suggest excess CO₂ (poor ventilation, closed room, basement tank) or low alkalinity.

If pH is chronically low, improve air exchange around the tank before adjusting chemically. Opening a window near the tank or running an airline from outdoors to the skimmer intake can raise pH by 0.1 to 0.3 units. Kalkwasser dosing also raises pH as a secondary benefit.

pH is best monitored with a continuous probe and controller. Spot testing with liquid kits is useful but misses the daily cycle. If your alkalinity is stable and within range, pH usually takes care of itself.

Calcium

Calcium is consumed by stony corals and coralline algae to build aragonite skeletons. The target range is 380 to 450 ppm, with 420 ppm matching natural seawater. In SPS-heavy tanks, calcium consumption can exceed 20 ppm per day, requiring daily dosing through two-part solutions, a calcium reactor, or kalkwasser.

Calcium and alkalinity must be managed together. Raising one without the other creates conditions for precipitation, which crashes both values simultaneously. Always test both before making adjustments. For detailed dosing guidance, see Calcium in Reef Tank.

Alkalinity

Alkalinity measures the water's buffering capacity, primarily the concentration of carbonate and bicarbonate ions. It is the most dynamically consumed parameter in reef tanks and the one most closely linked to coral calcification rate. The target range is 7.0 to 10.0 dKH, with 7.5 to 9.0 dKH being the most commonly recommended window.

Alkalinity swings are one of the most common causes of SPS stress. In high-demand tanks, testing every two to three days (or daily when dialing in dosing) is advisable. Stability within a 0.5 dKH band is the goal. For a complete breakdown, see Alkalinity Explained.

Magnesium

Magnesium stabilizes calcium and alkalinity in solution by inhibiting spontaneous calcium carbonate precipitation. The target is 1250 to 1350 ppm. When magnesium drops below 1200 ppm, calcium and alkalinity become unstable regardless of dosing accuracy.

Magnesium consumption is slow compared to calcium and alkalinity. Monthly testing is sufficient in most tanks. Coralline algae is the largest biological consumer. If you are troubleshooting calcium and alkalinity instability, test magnesium first. See Magnesium in Reef Tank.

Nitrate

Nitrate is the endpoint of the nitrogen cycle and a key nutrient for coral zooxanthellae. The ideal range is 1 to 10 ppm. Too low (below 0.5 ppm) and corals pale from nitrogen starvation. Too high (above 20 ppm) and corals brown while nuisance algae proliferates.

Nitrate is managed through feeding control, protein skimming, water changes, and optional methods like carbon dosing or refugium algae growth. The ratio between nitrate and phosphate matters as much as the absolute value. See Nitrate Levels in Reef.

Phosphate

Phosphate inhibits calcification and fuels algae growth at elevated levels. The target is 0.01 to 0.10 ppm. Like nitrate, phosphate at zero is harmful because zooxanthellae need phosphorus for basic cellular functions.

GFO (granular ferric oxide) is the most common removal method. Carbon dosing reduces both phosphate and nitrate together. Rapid phosphate drops can trigger tissue necrosis in SPS corals, so changes should be gradual. See Phosphate in Reef Tank.

Ammonia and Nitrite

Both should be undetectable (0 ppm) in a cycled reef tank. Any detectable reading indicates a cycling issue, a die-off event, or a sudden bioload increase. Test immediately if livestock shows stress (rapid breathing, loss of appetite, unusual behavior).

If ammonia or nitrite is detected, perform an immediate 20 to 25% water change and identify the source. Common causes include a dead fish or invertebrate hidden in the rockwork, a filter media replacement that removed too much beneficial bacteria, or a new tank that has not fully cycled.

Potassium

Potassium is consumed by soft corals, macroalgae, and some LPS species. Most salt mixes provide adequate potassium, and water changes replenish it. In tanks with heavy soft coral or macroalgae growth, potassium can deplete below 380 ppm, which may affect coloration and growth.

Test monthly or include it in quarterly ICP analysis. Potassium chloride supplements are available for correction if needed.

Strontium and Trace Elements

Strontium, iodine, iron, boron, and dozens of other trace elements play roles in biological processes. In most reef tanks, regular water changes replenish trace elements adequately. ICP testing every three to six months provides visibility into trace element levels without requiring individual test kits.

Do not dose trace elements blindly. Only supplement what you have tested and confirmed to be low. Overdosing trace elements is more common and more harmful than deficiency in hobby reef tanks.

Ranges by Coral Type

Different coral types have different sensitivities and optimal ranges. Here is how the most important parameters shift based on coral mix.

SPS-Dominant Reef

SPS corals are the most parameter-sensitive group. They calcify rapidly and respond to chemistry shifts faster than other coral types.

Parameter SPS Target
Calcium 420 to 450 ppm
Alkalinity 7.5 to 9.0 dKH
Magnesium 1280 to 1350 ppm
Nitrate 1 to 5 ppm
Phosphate 0.01 to 0.05 ppm
Temperature 76 to 78°F

Stability is paramount. SPS corals respond to alkalinity swings of even 0.5 dKH with tissue recession or color loss. Daily testing of alkalinity and calcium is recommended during the first months of an SPS system. For detailed SPS care, see SPS Coral Care.

LPS-Dominant Reef

LPS corals are more forgiving than SPS but still require stable water chemistry. They tolerate slightly higher nutrients and wider parameter swings.

Parameter LPS Target
Calcium 400 to 440 ppm
Alkalinity 7.0 to 9.5 dKH
Magnesium 1250 to 1350 ppm
Nitrate 5 to 15 ppm
Phosphate 0.03 to 0.10 ppm
Temperature 76 to 80°F

Most LPS species prioritize feeding over photosynthesis for energy, so slightly elevated nutrients support their feeding biology without the browning effect seen in SPS.

Soft Coral Tank

Soft corals are the most tolerant of parameter variation. They do not build calcium carbonate skeletons, so calcium and alkalinity consumption is minimal (though coralline algae in the same tank still consumes both).

Parameter Soft Coral Target
Calcium 380 to 420 ppm
Alkalinity 7.0 to 10.0 dKH
Magnesium 1200 to 1350 ppm
Nitrate 5 to 20 ppm
Phosphate 0.05 to 0.15 ppm
Temperature 76 to 80°F

In soft coral tanks, the primary chemistry concerns are salinity stability and avoiding extreme nutrient buildup. Dosing calcium and alkalinity is often unnecessary beyond what water changes provide.

Testing Schedule

Knowing what to test and how often prevents both over-testing (wasting reagents and creating anxiety) and under-testing (missing problems before they become visible).

Here is a practical testing schedule for an established reef tank:

Frequency Parameters
Daily Temperature (via controller or thermometer), salinity (visual check on ATO function)
Twice weekly Salinity (refractometer confirmation)
Weekly Calcium, alkalinity, nitrate, phosphate
Monthly Magnesium, potassium (if soft coral heavy)
Quarterly ICP analysis (strontium, iodine, trace elements, heavy metals)
As needed Ammonia, nitrite (only if livestock shows stress or after a die-off event)

When first setting up a reef tank or dialing in a new dosing routine, test calcium and alkalinity daily for the first two weeks. Once consumption is established and dosing is dialed in, reduce to weekly testing.

How Parameters Interact

No reef parameter exists in isolation. The relationships between parameters determine overall system stability, and managing them individually without understanding the connections leads to frustration.

The Calcium-Alkalinity-Magnesium Triad

These three parameters form the foundation of reef mineral chemistry. Calcium and alkalinity are consumed together during calcification. Magnesium prevents them from precipitating spontaneously. If magnesium is low, calcium and alkalinity cannot be maintained. If calcium is dosed without alkalinity (or vice versa), the imbalance triggers precipitation that crashes both.

The practical rule: always check all three before adjusting any one. Correct magnesium first if it is low. Then balance calcium and alkalinity together.

The Nitrate-Phosphate Nutrient Balance

Both are consumed by zooxanthellae and algae. Their ratio influences coral coloration and health more than either value alone. An imbalance (one very high, the other very low) causes more problems than both being moderately elevated. Manage them together, especially when using carbon dosing or GFO, which can selectively remove one faster than the other.

Salinity and Everything Else

Salinity sets the baseline concentration for every ion. A 1 ppt salinity change shifts all ion concentrations proportionally. If calcium reads 400 ppm but salinity has risen to 37 ppt, the actual calcium availability is different from what it would be at 35 ppt. Always verify salinity is stable before interpreting other parameter readings.

pH, Alkalinity, and CO₂

pH is determined by the balance between alkalinity (which buffers pH up) and dissolved CO₂ (which drives pH down). Low pH with normal alkalinity indicates excess CO₂, not an alkalinity problem. High pH with normal alkalinity indicates low CO₂ (good air exchange or kalkwasser dosing). Chasing pH by adding buffers without understanding the CO₂ relationship usually creates more instability.

Advanced: Natural Seawater as a Benchmark

Natural seawater composition provides the baseline that all reef parameter targets are derived from. Here are the precise values for average tropical ocean water at 35 ppt salinity:

Parameter Natural Seawater Value
Salinity 35.0 ppt
pH 8.1 to 8.3
Calcium 420 ppm
Alkalinity 7.0 dKH (2.5 meq/L)
Magnesium 1280 ppm
Potassium 399 ppm
Strontium 8.0 ppm
Sodium 10,780 ppm
Chloride 19,350 ppm
Sulfate 2,660 ppm
Boron 4.4 ppm
Nitrate < 1 ppm
Phosphate < 0.01 ppm

You will notice that natural seawater has lower alkalinity, nitrate, and phosphate than many reef keepers target. This is because reef tanks are closed systems with higher bioload per volume than a natural reef. The slightly elevated targets for hobby reef tanks account for the practical realities of feeding, waste production, and the difficulty of maintaining truly ocean-level nutrient concentrations in a closed system.

Natural seawater alkalinity of 7.0 dKH is worth noting specifically. Many reef keepers run alkalinity at 8 to 9 dKH to provide a larger buffer against consumption between dosing intervals. This works well in practice, but pushing alkalinity above 10 dKH in pursuit of "more growth" increases precipitation risk without meaningful calcification benefit.

Advanced: ICP Testing and When It Matters

ICP-OES (inductively coupled plasma optical emission spectrometry) testing analyzes a water sample for 30 or more elements simultaneously. It reveals trace element concentrations, heavy metal contamination, and ionic drift that individual test kits cannot detect.

ICP testing is most valuable in these situations:

  • Quarterly checkups on established tanks to catch slow drift in trace elements or accumulation of heavy metals (copper, zinc, nickel) from equipment or supplements.
  • Troubleshooting unexplained coral stress when all standard parameters test normal. Elevated copper from a corroding fitting or depleted iodine from heavy skimming may only show up on ICP.
  • Evaluating a new salt mix to verify its actual ionic composition matches the manufacturer's claims.
  • After equipment changes (new pump, heater, plumbing) to check for metal leaching.

ICP does not replace regular testing of core parameters. It complements it. The turnaround time (typically one to two weeks) makes ICP unsuitable for day-to-day management. Use your weekly test kit results for routine management and ICP for periodic deep analysis.

Common Myths

"You need to match natural seawater exactly." Natural seawater is the reference point, not the requirement. Reef tanks are closed systems with different dynamics. Slightly elevated alkalinity and nutrients are practical adaptations that work well. Obsessing over exact NSW values creates unnecessary complexity.

"More testing means better results." Testing beyond what is actionable creates anxiety without improving outcomes. Weekly testing of core parameters is sufficient for established tanks. Daily testing is only needed during initial setup or dosing calibration.

"All salt mixes are the same at 35 ppt." Salt mixes vary significantly in calcium, alkalinity, magnesium, and trace element concentrations. Some are designed for higher-than-natural calcium and alkalinity. Others target NSW ratios. The best salt mix is one that matches your system's needs and produces consistent results batch to batch.

"If one parameter is off, fix it immediately." Rapid corrections cause more harm than the original deviation. A calcium reading of 380 ppm does not require emergency dosing. Gradual correction over one to two days is safer and more effective. The only parameters requiring immediate action are ammonia, nitrite, and extreme salinity deviations.

"ICP testing replaces hobby test kits." ICP provides a comprehensive snapshot but takes one to two weeks for results. It cannot replace weekly calcium, alkalinity, and nitrate monitoring. Use both: ICP for deep analysis, hobby kits for ongoing management.

FAQ

What are the most important parameters to test in a reef tank?

Salinity, temperature, calcium, alkalinity, and nitrate/phosphate are the core parameters. Magnesium should be checked monthly. These cover the critical chemistry that affects coral health and growth.

How often should I test my reef tank water?

Weekly testing of calcium, alkalinity, nitrate, and phosphate is sufficient for established tanks. Check salinity twice per week. Test magnesium monthly. During initial setup or when adjusting dosing, test calcium and alkalinity daily.

What is the ideal alkalinity for a reef tank?

The target range is 7.0 to 10.0 dKH, with 7.5 to 9.0 dKH being the most commonly recommended. Stability within a narrow range matters more than the exact number.

Do I need to test for trace elements?

Not with individual test kits in most cases. Quarterly ICP analysis provides a comprehensive trace element profile. Regular water changes replenish trace elements in most tanks without supplementation.

What is the Redfield ratio and why does it matter?

The Redfield ratio describes the approximate 16:1 nitrogen-to-phosphorus ratio that marine organisms consume nutrients at. In practical reef terms, keeping nitrate at roughly 100 times the phosphate concentration (by ppm) maintains a healthy nutrient balance for zooxanthellae.

Should I chase natural seawater values exactly?

No. NSW values are the reference point, but reef tanks are closed systems with higher bioload. Slightly elevated alkalinity (8 to 9 dKH vs. NSW 7 dKH) and nutrients (2 to 5 ppm nitrate vs. NSW < 1 ppm) are practical targets that account for closed-system dynamics.

What should I do if multiple parameters are off at the same time?

Address salinity first, then magnesium, then calcium and alkalinity together, then nutrients. Correcting in this order prevents compounding errors. Make each change gradually (over one to two days minimum) before adjusting the next parameter.

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