The Complete Guide to Reef Tank Water Parameters

Maintaining proper water chemistry is the foundation of successful reef keeping. Your corals, fish, and invertebrates depend on stable, balanced water parameters to thrive. Whether you’re just starting your first reef tank or looking to level up your husbandry, understanding these parameters is essential.

In this comprehensive guide, we’ll cover the 8 essential parameters every reef keeper should monitor, explain why each one matters, and give you practical strategies for maintaining the stability your reef needs to flourish.

Why Water Parameters Matter

Reef aquariums are closed systems that attempt to replicate the chemistry of natural seawater. In the ocean, the sheer volume of water maintains incredibly stable conditions—the same water chemistry that coral reefs evolved in over millions of years. In our tanks, we need to actively manage these parameters because:

• Corals are sensitive — Even small fluctuations can stress coral and inhibit growth. In nature, reef-building corals experience parameter changes measured in fractions over months, not points over days.
• Chemistry is interconnected — Parameters like calcium, alkalinity, and magnesium are linked in complex ways. Change one, and you’ll affect the others.
• Problems compound — Small issues become big problems if left unchecked. A slightly off parameter might cause minor stress today, but that stress weakens organisms and opens the door to disease, algae, and coral loss.
• Stability matters more than perfection — Consistent parameters beat “perfect” numbers that fluctuate. A tank running steady at 8.5 dKH alkalinity will outperform one that swings between 8 and 10 dKH chasing an “ideal” number.

The good news? Once you understand these relationships and establish good testing habits, maintaining water chemistry becomes routine rather than stressful.

The Essential Eight Parameters

Let’s dive deep into each of the eight parameters you should be monitoring in your reef tank.

1. Calcium (Ca) — Target: 420 ppm

Calcium is the primary building block of coral skeletons. Stony corals (SPS and LPS) actively extract calcium from the water to build their calcium carbonate structures. This process, called calcification, is how corals grow and create the reef structures we love.

Ideal Range: 380-450 ppm

Why It Matters:

• Essential for coral skeleton growth — Without adequate calcium, corals simply cannot grow. Their skeleton is made of calcium carbonate (CaCO3), and they need a steady supply of calcium ions to build it.
• Required by coralline algae, clams, and other calcifiers — It’s not just corals. Coralline algae (the purple/pink encrusting algae we want), Tridacna clams, certain tube worms, and other invertebrates all need calcium.
• Low calcium = slow growth and weak skeletons — Corals in calcium-deficient water grow slowly and produce thin, brittle skeletons more susceptible to damage.
• High calcium (over 500 ppm) can precipitate with alkalinity — Calcium and carbonates (alkalinity) are in a delicate balance. Push calcium too high and it can combine with carbonates to form calcium carbonate deposits, crashing both parameters and leaving a white “snowstorm” in your tank.

Testing Frequency: Weekly, or more often in heavily stocked SPS tanks. If you’re running a calcium reactor or dosing heavily, test 2-3 times per week until you understand your tank’s consumption rate.

Common Issues:

• Calcium won’t stay up: Usually indicates low magnesium or that you’re dosing alkalinity too heavily relative to calcium.
• Calcium rising unexpectedly: Could be overdosing, or if paired with dropping alkalinity, indicates an imbalance in your two-part dosing ratio.
• Calcium stuck at 500+ ppm: Ease off dosing and let your corals consume it down. Don’t add more until you’re in the 380-450 range.

2. Alkalinity (Alk) — Target: 9.0 dKH

Alkalinity measures the water’s buffering capacity—its ability to resist pH changes. More practically, it provides the carbonate and bicarbonate ions that corals need for calcification. Alkalinity is the most consumed parameter in a reef tank and the one most likely to cause problems if neglected.

Ideal Range: 7-11 dKH (many SPS keepers prefer 7.5-8.5 dKH for coloration)

Why It Matters:

• Most consumed parameter in reef tanks — Your tank uses alkalinity faster than any other major element. A tank full of SPS corals can consume 1+ dKH per day.
• Directly affects calcification rate — Higher alkalinity (within reason) generally means faster coral growth. Lower alkalinity means slower growth.
• Buffers pH stability — Alkalinity acts as a buffer against pH swings. Low alkalinity = unstable pH.
• Low alk = pH swings and slow growth — When alkalinity drops too low, you’ll see pH fluctuations and stalled coral growth.
• Rapid alk changes are extremely stressful to corals — This is critical. A sudden drop or spike in alkalinity is one of the fastest ways to kill SPS corals. Never change alkalinity by more than 1 dKH per day, and slower is always better.

Testing Frequency: Every 1-3 days for SPS tanks, weekly for mixed reefs and soft coral systems. Alkalinity testing should be your most frequent test.

Common Issues:

• Alkalinity dropping too fast: Either heavy coral consumption (good problem!) or something in your system is consuming alk. Check your calcium reactor effluent, kalkwasser mixer, or two-part dosing rate.
• Alkalinity won’t come up: Low magnesium is often the culprit. Also check that your test kit is accurate—Hanna checkers are gold standard.
• Coral STN/RTN after alk adjustment: You changed alkalinity too fast. In the future, never adjust by more than 0.5-1 dKH per day.

3. Magnesium (Mg) — Target: 1350 ppm

Magnesium prevents calcium and alkalinity from precipitating out of solution. Think of it as the “glue” that holds your chemistry together. While often overlooked, magnesium issues are frequently the hidden cause of calcium and alkalinity instability.

Ideal Range: 1250-1400 ppm

Why It Matters:

• Enables stable calcium/alkalinity relationship — Without adequate magnesium, calcium and alkalinity will readily combine and precipitate out of solution, crashing both levels.
• Low magnesium = difficulty maintaining calc/alk — If you’re fighting to keep calcium and alkalinity stable, check magnesium first. It’s often the answer.
• Required by some organisms directly — Certain coralline algae species and invertebrates use magnesium directly.
• Generally stable and consumed slowly — Unlike alkalinity, magnesium consumption is slow. Once you get it in range, it usually stays there with regular water changes.

Testing Frequency: Weekly to bi-weekly. Monthly is fine once you know your tank’s stability.

Common Issues:

• Magnesium low despite dosing: Could be excessive precipitation. Check for white deposits on pumps, heaters, and glass. Also verify your test kit with a reference solution.
• Calc/alk won’t stay stable: Check magnesium! This is the most common hidden problem.
• Magnesium too high (over 1500): Ease off dosing. Very high magnesium can inhibit calcification.

4. Salinity — Target: 1.026 SG

Salinity measures the total dissolved salts in your water. It affects the osmotic balance for all organisms in your tank and influences how other parameters behave.

Ideal Range: 1.024-1.027 specific gravity (32-35 ppt)

Why It Matters:

• Affects all chemical processes — Higher salinity means higher concentrations of calcium, alkalinity, and every other ion. This affects dosing calculations and test results.
• Organisms are adapted to specific salinity ranges — Fish, corals, and invertebrates have evolved for a specific osmotic pressure. Sudden changes stress their cellular processes.
• Rapid changes cause osmotic stress — Fish will “flash” and act stressed. Corals may retract or shed mucus. Invertebrates can be severely affected.
• Evaporation concentrates salinity — As water evaporates, salt stays behind, raising salinity. Always top off with fresh RODI water, never saltwater.

Testing Frequency: Daily visual check of your auto-top-off (ATO) system, weekly calibrated reading with a refractometer. If you don’t have an ATO, check daily.

Common Issues:

• Salinity creeping up: ATO not keeping up with evaporation, or reservoir empty. Check your ATO system.
• Salinity dropping: ATO malfunction flooding fresh water, or you’re accidentally doing water changes with lower-salinity water.
• Inconsistent readings: Refractometer needs calibration. Use calibration fluid (not RO water) and recalibrate monthly.

5. pH — Target: 8.2

pH measures how acidic or alkaline your water is on a logarithmic scale. Reef tanks need to stay alkaline, and pH affects everything from coral calcification to the toxicity of ammonia.

Ideal Range: 8.0-8.4

Why It Matters:

• Affects biological processes and calcification — Higher pH (up to 8.4) promotes faster calcification. Lower pH slows growth and can even dissolve coral skeletons over time.
• Influenced by CO2 levels, alkalinity, and bioload — pH is the result of many factors. High CO2 (from closed houses in winter or heavy fish loads) lowers pH. Good alkalinity helps buffer pH.
• Natural daily fluctuation is normal — pH naturally rises during the day when lights are on (photosynthesis consumes CO2) and drops at night. A 0.2-0.3 swing is normal.
• Stability matters more than hitting an exact number — A tank stable at 7.9-8.1 is healthier than one swinging between 7.8 and 8.4.

Testing Frequency: Continuous monitoring with a pH probe is ideal. Otherwise, weekly testing at the same time of day. Test both morning (lights off) and evening (lights on) to understand your range.

Common Issues:

• pH chronically low (below 8.0): Usually CO2 related. Try opening windows, running a skimmer air line outside, or adding a CO2 scrubber to your skimmer intake. Also check alkalinity.
• pH too high (above 8.5): Usually from excessive kalkwasser dosing or new aragonite sand/rock outgassing.
• Large pH swings: Improve alkalinity stability, increase surface agitation, or address CO2 sources.

Pro tip: Running a refugium on a reverse light cycle (lights on at night) helps stabilize pH by having something photosynthesizing 24/7.

6. Temperature — Target: 78°F (25.5°C)

Reef organisms are adapted to tropical temperatures. They’re cold-blooded, meaning their metabolism depends on water temperature. Stability here is critical.

Ideal Range: 76-80°F (24-27°C). Some keepers run 75-78°F to reduce metabolism and slow disease progression.

Why It Matters:

• Affects metabolism, oxygen levels, and disease resistance — Higher temps speed up metabolism (corals grow faster but consume more) and reduce dissolved oxygen. Diseases also progress faster in warmer water.
• Temperature swings (over 2°F daily) stress organisms — Large swings are worse than a slightly non-ideal stable temperature.
• High temps reduce oxygen and can trigger bleaching — Above 82°F, you’re in the danger zone. Coral bleaching (expelling zooxanthellae) can occur during heat waves.
• Consistent temperature supports consistent chemistry — Temperature affects how other parameters behave and how fast biological processes occur.

Testing Frequency: Continuous monitoring with a heater/chiller controller is essential. At minimum, check a reliable thermometer twice daily.

Common Issues:

• Temperature swings: Usually inadequate heater/chiller capacity, heater stuck on, or ambient room temperature changes. Use redundant temperature control if possible.
• Chronic high temps: Add a fan, chiller, or move lights higher. Consider LED lights which produce less heat.
• Heater failure: Always use a separate temperature controller as a backup cutoff. A stuck-on heater will cook your tank.

7. Nitrate (NO₃) — Target: 5 ppm

Nitrate is the end product of the nitrogen cycle. Your biological filtration converts toxic ammonia to nitrite, then to nitrate. It’s a nutrient that feeds coral and algae alike.

Ideal Range: 1-10 ppm (varies by methodology—some SPS keepers go lower, some reef tanks thrive at 15-20 ppm)

Why It Matters:

• Too low = pale, starving coral — Corals need nutrients. Ultra-low-nutrient systems (ULNS) can result in pale, slow-growing corals, especially LPS and soft corals that rely more heavily on dissolved nutrients.
• Too high = algae problems and potential coral stress — Excess nitrate feeds nuisance algae and can inhibit coral calcification. Above 40-50 ppm, you’ll likely see problems.
• Ultra-low nutrient systems (ULNS) aren’t for everyone — While some advanced SPS keepers run near-zero nitrates, this requires pristine husbandry and isn’t ideal for beginners.
• Balance with phosphate is key — Nitrate and phosphate should be kept in balance. Extremely low nitrate with detectable phosphate (or vice versa) causes problems.

Testing Frequency: Weekly. More often if you’re actively trying to adjust levels.

Common Issues:

• Nitrates too high: Overfeeding, inadequate export (skimming, water changes), or dead spot with detritus buildup. Consider adding a refugium with macroalgae.
• Nitrates too low: Heavy skimming, carbon dosing, or simply not enough feeding. Try reducing export or feeding more.
• Nitrates won’t come down: Check phosphate (can be bound in rock), reduce feeding, increase water changes, or try a refugium.

8. Phosphate (PO₄) — Target: 0.05 ppm

Phosphate is another nutrient that affects coral growth, coloration, and algae. It’s often the trickier of the two nutrients to control.

Ideal Range: 0.02-0.1 ppm (some keepers run 0.1-0.2 with excellent results)

Why It Matters:

• Too high = inhibits calcification, promotes algae — Phosphate directly interferes with the calcification process and is premium fertilizer for nuisance algae.
• Too low = nutrient deficiency, pale coral — Just like nitrate, phosphate is a nutrient corals need. Zero phosphate leads to pale, unhealthy corals.
• Harder to remove than nitrate — Phosphate binds to rock and substrate, creating a “reserve” that leaches back. This makes it harder to control.
• Ratio with nitrate matters — The Redfield ratio suggests ~16:1 N:P by atoms, which works out to roughly 100:1 by ppm (since we measure nitrate-nitrogen differently). If your nitrate is 10 ppm, aim for ~0.1 ppm phosphate.

Testing Frequency: Weekly. Use a Hanna phosphate checker for accuracy—standard test kits often aren’t sensitive enough at reef-keeping levels.

Common Issues:

• Phosphate too high: Overfeeding, phosphate-leaching rock, or inadequate export. Try GFO (granular ferric oxide), reduce feeding, or increase water changes.
• Phosphate reading zero but algae problems: Algae may be consuming phosphate as fast as it’s produced. The rock may also be releasing bound phosphate. Try reducing nitrate too.
• Phosphate won’t stay down: Your rock is likely leaching stored phosphate. This can take months to resolve with consistent GFO and water changes.

The Calcium-Alkalinity-Magnesium Triangle

These three parameters are interconnected in a relationship reef keepers call the “ionic balance” or the “Cal/Alk/Mag triangle.” Understanding this relationship is key to maintaining stable chemistry.

How They’re Connected

1. Calcium and alkalinity are consumed together — During calcification, corals use both calcium ions (Ca²⁺) and carbonate ions (CO₃²⁻, which comes from your alkalinity). They’re consumed in a relatively fixed ratio.

2. Magnesium must be in range for the relationship to stay stable — Magnesium ions interfere with the crystal structure of calcium carbonate, making it harder for calcium and alkalinity to spontaneously precipitate. Low magnesium = easy precipitation.

3. If one is off, the others will be affected — They exist in chemical equilibrium. You can’t change one without influencing the others.

4. Dose in balance — When supplementing, you need to add calcium and alkalinity in approximately the ratio your tank consumes them. Adding only one will throw off the balance.

Practical Guidelines

• Always check magnesium first when troubleshooting calc/alk issues
• Use balanced dosing methods (two-part, calcium reactor, or kalkwasser) rather than trying to dose individual elements
• Track consumption rates to understand your tank’s needs
• Adjust slowly — Never try to fix all parameters at once

The NSW (Natural Seawater) Baseline

Natural seawater has approximately:

• Calcium: 420 ppm
• Alkalinity: 7 dKH
• Magnesium: 1280 ppm

Many reef keepers successfully run slightly elevated levels (especially alkalinity) to promote faster growth, but natural seawater values are a perfectly valid target.

Testing Tips for Accurate Results

The best parameter targets mean nothing if your test results aren’t accurate. Here’s how to ensure you’re getting reliable data:

Choose Quality Test Kits

Not all test kits are created equal. For critical parameters:

• Alkalinity: Hanna dKH checker (HI772) is the gold standard
• Calcium: Hanna calcium checker (HI758) or Salifert test kit
• Magnesium: Salifert or Red Sea
• Phosphate: Hanna ULR phosphate checker (HI736)—standard kits aren’t sensitive enough
• Nitrate: Hanna nitrate checker or Red Sea
• Salinity: A quality refractometer calibrated with 35 ppt solution (not RO water)

Testing Best Practices

• Consistency matters — Test at the same time of day for comparable results
• Calibrate regularly — pH probes monthly, refractometers monthly, verify test kits with reference solutions periodically
• Fresh reagents — Replace test kit reagents annually or as directed. Old reagents give bad results.
• Follow instructions exactly — Timing matters. Shaking matters. Temperature of reagents matters.
• Track your results — Use ReefTanker to see trends over time. Trends tell you more than individual readings.
• Test your salt mix — Know what your water change water provides so you can account for it
• Test freshly mixed saltwater — Let it mix and stabilize 24 hours before testing or using

When Results Don’t Make Sense

If you get a reading that seems wrong:

1. Re-test with fresh sample water
2. Check reagent expiration dates
3. Verify with a second test kit or method
4. Have your LFS test the same sample
5. Consider ICP testing for a laboratory-verified baseline

Maintaining Stability: The Real Secret

The secret to reef success isn’t hitting perfect numbers—it’s maintaining stability. Corals can adapt to a range of conditions, but they struggle with constant change.

Building a Stable System

1. Test regularly and track results — You can’t manage what you don’t measure. Testing reveals trends before they become problems.

2. Make small adjustments — Never change parameters rapidly. If alkalinity is at 6 dKH and you want 8 dKH, take a week or more to get there.

3. Automate what you can — Dosing pumps, auto-top-off systems, and controllers remove human error and provide consistency.

4. Understand consumption rates — Know how fast your tank uses calcium and alkalinity. This tells you how much to dose.

5. Water changes help — Regular water changes (10-20% weekly or bi-weekly) replenish trace elements, export waste, and help reset chemistry.

6. Don’t chase numbers — If your corals are thriving at 8 dKH, don’t try to push to 9 because someone on a forum said that’s “better.”

7. Change one thing at a time — If you need to make adjustments, change one parameter and wait to see the effect before changing another.

Warning Signs to Watch For

• Parameters drifting despite consistent dosing — Something changed. New coral growth? Equipment malfunction? Contamination?
• Unexplained coral stress — Test everything, including trace elements via ICP if needed
• Inability to keep calc/alk stable — Almost always a magnesium issue
• Test results that don’t match coral health — Your tests might be wrong. Verify with other methods.

Creating Your Testing Schedule

Based on the guidelines above, here’s a suggested testing schedule:

Daily

• Visual check of ATO reservoir and operation
• Temperature monitoring (continuous if possible)
• Observe livestock for behavior changes

Every 2-3 Days

• Alkalinity (SPS-dominant tanks)

Weekly

• Alkalinity (mixed reef/soft coral tanks)
• Calcium
• Magnesium
• Nitrate
• Phosphate
• Salinity (calibrated refractometer reading)

Monthly

• Full parameter review and trend analysis
• Calibrate pH probe and refractometer
• Consider ICP test quarterly to verify trace elements

Next Steps

Now that you understand the essential parameters, start tracking them consistently. The patterns in your data will reveal how your tank behaves, what it consumes, and guide your maintenance decisions.

Key takeaways:

1. Stability over perfection — Consistent parameters matter more than “perfect” numbers
2. The Big Three are connected — Calcium, alkalinity, and magnesium work together
3. Test accurately and regularly — Good data drives good decisions
4. Track trends over time — Single readings mean less than patterns
5. Adjust slowly — Patience prevents disasters

Ready to start tracking? Try ReefTanker to log your parameters, visualize trends, and master your reef chemistry. Our app makes it easy to spot trends, set reminders for testing, and understand your tank’s unique needs.

Have questions about reef tank water parameters? Contact us or follow us on social media for more tips and guides.