Reef Water Chemistry Guide
Reef tanks are chemical systems as much as they are biological ones. Corals build their skeletons from calcium carbonate, a process that continuously draws alkalinity, calcium, and magnesium from the water. Fish and feeding add ammonia, nitrate, and phosphate. Evaporation concentrates everything that doesn’t evaporate. In a closed system, these processes shift parameters constantly — and corals are sensitive to those shifts in ways that fish are not.
This guide covers the key water chemistry parameters in a reef tank, what they mean, why they matter, and how to maintain them.
The Big Three: Alkalinity, Calcium, and Magnesium
These three parameters are the foundation of coral chemistry. Corals use alkalinity and calcium together to build calcium carbonate skeletons. Magnesium stabilizes the relationship between the two. In an active reef tank, all three are consumed continuously and must be replenished through water changes, dosing, or a calcium reactor.
Alkalinity (dKH)
Alkalinity — measured in dKH (degrees of carbonate hardness) or meq/L — is the most important parameter to monitor and maintain in a reef tank. It represents the water’s capacity to buffer against pH changes and is the primary building block corals use for skeletal growth alongside calcium.
- Target range: 7.5–9 dKH for most mixed reef tanks. Stability within your chosen range matters more than the exact number.
- Why it matters: Alkalinity swings of more than 0.5–1 dKH per day can stress or kill corals. Consistent alkalinity is more important than hitting a specific target.
- How it’s consumed: Corals consume alkalinity as they grow. Test at least twice per week in active reef tanks.
- How to maintain it: Regular water changes with quality reef salt replenish alkalinity. Dosing (two-part, kalkwasser, or calcium reactor) is used when consumption exceeds what water changes can replace.
Calcium (ppm)
Calcium is used alongside alkalinity for coral skeletal growth.
- Target range: 380–450 ppm. Natural seawater is approximately 420 ppm.
- Why it matters: Low calcium slows coral growth. Very high calcium (above 500 ppm) can cause precipitation, removing both calcium and alkalinity simultaneously.
- Relationship with alkalinity: Calcium and alkalinity are chemically linked — large imbalances between the two can indicate a dosing or mixing problem.
- How to maintain it: Water changes and dosing. Two-part dosing replenishes both calcium and alkalinity in a balanced ratio.
Magnesium (ppm)
Magnesium stabilizes the relationship between calcium and alkalinity. Without adequate magnesium, calcium carbonate precipitates more readily.
- Target range: 1250–1350 ppm. Natural seawater is approximately 1280 ppm.
- Why it matters: Low magnesium makes it difficult to maintain stable calcium and alkalinity. If either keeps dropping despite dosing, check magnesium first.
- How it’s consumed: Much more slowly than alkalinity or calcium. Monthly testing is usually sufficient in established tanks.
- How to maintain it: Water changes with quality reef salt typically maintain adequate magnesium. Supplements are available when correction is needed.
Salinity and Specific Gravity
- Target range: 1.025–1.026 specific gravity (approximately 33–35 ppt). Natural seawater is approximately 35 ppt.
- Why it matters: Salinity affects the osmotic balance of all marine organisms and the accuracy of other parameter measurements.
- How it changes: Evaporation raises salinity because only water evaporates — salt stays behind. An ATO replenishing evaporation with fresh RO/DI water is the most reliable way to maintain stable salinity.
- How to measure it: Use a refractometer calibrated with calibration solution, or a digital salinity meter. Swing-arm hydrometers are not accurate enough for reef tanks.
pH
Reef tanks target a pH of 8.1–8.3, approximating natural seawater.
- Why it matters: pH affects coral calcification rates. Chronically low pH (below 7.8–8.0) can slow coral growth and stress livestock.
- Natural fluctuation: pH rises during the day as photosynthesis consumes CO₂, and falls at night. A swing of 0.1–0.2 pH units over 24 hours is normal. A continuous pH monitor is more useful than a point-in-time test.
- Common causes of low pH: High CO₂ in the home, insufficient surface agitation, or a refugium running on the same light cycle as the display tank. Running the refugium on a reverse photoperiod helps stabilize pH.
- Caution: Chasing pH with additives is rarely recommended unless the underlying cause has been identified. Correcting CO₂ levels and improving gas exchange is more effective.
Nutrients: Nitrate and Phosphate
Nitrate (ppm)
- Target range: 1–10 ppm for most mixed reef tanks. SPS systems often target 1–5 ppm or lower. Zero nitrate is not ideal.
- How it accumulates: End product of the nitrogen cycle. Must be exported through water changes, refugium, algae scrubber, or carbon dosing.
- Signs of excess: Nuisance algae growth, reduced coral color and polyp extension.
Phosphate (ppm)
- Target range: 0.03–0.1 ppm. Zero phosphate is not ideal — some is necessary for coral health.
- How it accumulates: From feeding and organic decomposition, and from tap water — another reason RO/DI water is essential.
- Signs of excess: Nuisance algae, inhibited coral calcification, reduced coral color.
- How to manage it: Protein skimming, refugium, GFO in a media reactor, and careful feeding. Use a ULR-capable phosphate test kit — standard kits often can’t resolve the difference between 0.02 and 0.08 ppm.
Temperature
Reef tanks target 76–80°F (24–27°C).
- Why it matters: Temperatures above 82–84°F for extended periods trigger bleaching in many coral species.
- Stability: A tank that holds steady at 79°F is healthier than one that swings between 76°F and 82°F daily.
- Equipment: Use a quality heater with a separate thermometer or controller for redundancy. A heater that fails open can rapidly overheat a tank.
How to Maintain Stable Parameters
Water Changes
Regular water changes with quality reef salt are the simplest and most reliable way to replenish alkalinity, calcium, magnesium, and trace elements while exporting nitrate and phosphate. A 10–20% water change every 2–4 weeks is standard for most reef tanks. Always mix saltwater with RO/DI water, verify parameters before use, and match temperature and salinity to the tank before adding new water.
Dosing
When coral consumption exceeds what water changes can replenish, supplemental dosing is required. Common methods:
- Two-part dosing: Separate alkalinity and calcium solutions dosed in equal volumes. Simple, precise, and scalable. The most common approach for home reef tanks.
- Kalkwasser (limewater): Calcium hydroxide dissolved in RO/DI water, added via the ATO or drip. Replenishes calcium and alkalinity simultaneously and raises pH. Requires careful management to avoid overdosing.
- Calcium reactor: Circulates tank water through calcium carbonate media dissolved by CO₂. Highly effective for large, high-demand systems. More complex to set up than two-part dosing.
Water Chemistry FAQs
What parameters should I test first?
Start with salinity, temperature, and alkalinity — these three have the most immediate impact on coral and fish health. Once stable, add calcium, magnesium, nitrate, and phosphate to your routine. See our Best Reef Test Kits guide for equipment recommendations.
Why do my parameters keep changing?
Parameters change because corals consume alkalinity, calcium, and magnesium as they grow; fish and feeding add nitrate and phosphate; and evaporation concentrates everything that doesn’t evaporate. This is normal — the goal is to replenish what’s consumed and export what accumulates through a consistent maintenance routine.
What is two-part dosing?
Two-part dosing uses two separate solutions — one for alkalinity and one for calcium — dosed in equal volumes to replenish both parameters simultaneously. It’s the most common supplemental dosing method for home reef tanks because it’s simple, precise, and doesn’t require CO₂ equipment. Automated dosing pumps make it hands-off once dialed in.
Can I use tap water for water changes?
Not recommended. Tap water contains chlorine, chloramine, phosphates, nitrates, silicates, and dissolved minerals that accumulate over time. Always use 0 TDS RO/DI water. See our Best RO/DI Systems guide for recommendations.
What causes parameter swings and how do I prevent them?
The most common causes are infrequent water changes, inconsistent dosing, changes in coral growth rate, and equipment failures. Prevent swings by testing regularly, maintaining a consistent schedule, and monitoring equipment. Automated dosing pumps and ATOs significantly reduce variability.