Alkalinity — the buffer beneath the surface

Alkalinity is not just another parameter among many. It is the water’s ability to resist pH change — a buffer that protects all biological activity from chemical shock. Without it, the tank is unstable. Corals do not grow. pH crashes at night.

This article covers one of the most central parameters in the entire hobby: alkalinity. Calcium and magnesium have their own articles — but alkalinity should be understood first, because it is the most directly tied to pH and the immediate wellbeing of corals.

1. What is alkalinity, really?

Alkalinity means the water’s ability to bind acids — technically its acid-neutralising capacity. In a marine aquarium, this capacity comes almost entirely from bicarbonate ions (HCO₃⁻) and carbonate ions (CO₃²⁻) dissolved in the water.

When acid is added to the water — produced continuously by biological respiration and the nitrogen cycle — the buffer system absorbs the shock before pH drops. The more bicarbonate and carbonate in the water, the better it withstands this stress.

Alkalinity is generally measured in °dKH (German carbonate hardness degrees) or meq/l (milliequivalents per litre). Conversion: 1 meq/l = 2.8 °dKH. Most hobbyists use dKH.

Alkalinity is not pH

These two are confused more often than they should be. pH indicates the concentration of hydrogen ions (H⁺) on a logarithmic scale. Alkalinity indicates how much capacity the water has to resist pH change.

High alkalinity does not mean pH is high. Low alkalinity does not automatically mean low pH. They are connected — high alkalinity generally supports higher pH — but they are different things, managed by different means.

2. Why does alkalinity deplete?

In a closed aquarium system, alkalinity falls for two reasons:

Calcification. When corals build their skeletons, they take Ca²⁺ and CO₃²⁻ ions from the water and combine them into calcium carbonate (CaCO₃). Carbonate ions are part of alkalinity, so every millimetre of coral growth is also a small draw on alkalinity.

Biochemical acidification reactions. Respiration, bacterial activity and the breakdown of organic matter produce carbon dioxide (CO₂), which dissolves in water and forms carbonic acid (H₂CO₃). This acid is neutralised by the buffer system — so alkalinity is consumed whenever CO₂ is produced.

This means alkalinity falls over time in all active aquariums and must be actively replenished.

3. Reference values: where should you be?

In natural seawater, alkalinity is typically 6.5–7.2 °dKH. This is the biological baseline — the value at which corals have evolved to live.

In aquariums, the target range is slightly higher. The reason is practical: in a closed system, alkalinity fluctuates more readily than in the open ocean, and a slightly elevated level provides a buffer against short-term drops.

SPS tanks call for lower values. This contradicts old-school thinking where “more is better.” Research shows that many Acropora species grow better and remain stable at lower alkalinity — particularly when it stays stable. Severe fluctuations are more dangerous than a low but steady value.

More important than the absolute level is stability. Alkalinity that stays at 7.5–8.0 °dKH day after day is better than a value swinging between 7.0–9.5 weekly.

4. Alkalinity and pH — the night problem

One of alkalinity’s most important practical roles is limiting the nocturnal pH drop.

During the day, photosynthesis consumes CO₂ and raises pH. At night, photosynthesis stops but respiration continues — CO₂ begins to accumulate and pH falls. The more buffer capacity in the water (i.e. alkalinity), the slower the pH drops.

The practical consequence: low alkalinity means a larger nocturnal pH crash. If pH drops below 7.8 at night, corals may become stressed, calcification slows, and sensitive species may show symptoms the following day. Chronically repeated nocturnal drops accumulate biological harm over time.

Maintaining adequate alkalinity is one of the most effective ways to stabilise the pH cycle without separate pH management.

5. Measurement: how and when?

Home tests

Alkalinity is the easiest of the three major elements to measure at home:

• Salifert KH — simple titrimetry, one drop ≈ 0.1 °dKH, practical for routine measurement
• Red Sea KH Pro — wider scale, clear colour change with good lighting
• Hanna HI-772 Checker — photometer, digital readout, reduces subjectivity in colour interpretation

ICP laboratory

Alkalinity is included in all ICP laboratory packages. ICP is a good way to verify home test accuracy — batch-to-batch reagent variation is possible, and a systematic difference between home test and ICP result is worth catching early.

Measurement schedule

Measurement time affects the result. The best time to measure is the morning before lights come on and before feeding. This gives comparable results across measurement sessions.

6. Supplementation: how to keep alkalinity stable?

Four established methods — each with its own logic, suitability and limitations.

Water changes

The simplest starting point. Quality synthetic sea salt contains alkalinity, and regular water changes replenish it naturally. In small tanks or systems with few corals, a weekly 20–25% change is enough to keep alkalinity stable without separate dosing. When coral numbers grow and consumption exceeds what water changes can replace, one of the methods below is needed.

Kalkwasser (limewater)

Calcium hydroxide (Ca(OH)₂) is dissolved in RO/DI water, creating a clear alkaline solution — kalkwasser. The solution is dosed slowly into the tank, typically via an ATO system as evaporation replacement.

Key characteristics of kalkwasser:

• Raises both alkalinity and calcium simultaneously in a balanced ratio — no ionic imbalance
• Strong pH effect: the solution’s pH is extremely high, so it must be dosed slowly in small increments
• Well suited to small and medium tanks with low to moderate consumption — softies, LPS and light mixed reef systems
• In high-consumption SPS tanks, kalkwasser alone is generally insufficient

Two-part dosing (Balling-type systems)

Two-part dosing means simultaneously dosing two separate solutions — an alkalinity solution and a calcium solution — using dosing pumps. This is currently the most common method among hobbyists.

Basic logic: corals consume calcium and alkalinity in nearly the same molar ratio when building calcium carbonate. When both solutions are formulated to match this ratio, both deplete at the same rate and maintain balance.

The ionic balance problem with classic formulations: Traditional Balling solutions use sodium bicarbonate (NaHCO₃) as the alkalinity source and calcium chloride (CaCl₂) as the calcium source. Over time, this raises salinity and distorts ionic balance — the ion shift appears in ICP analysis after about 6–8 months.

Ionically balanced formulations solve the problem by integrating compensation elements — sulphate, potassium, bromide — directly into the base solutions.

Calcium media reactor

A calcium media reactor is a closed pressurised chamber reactor containing aragonite media. Tank water is pumped in and CO₂ is injected. CO₂ lowers the internal pH to about 6.5–6.8, dissolving aragonite media and releasing calcium and carbonate into the water.

A calcium media reactor provides automatic, continuous and ionically balanced supplementation without cumulative ion shift. Limitation: the reactor lowers tank pH because the effluent contains significant CO₂. Best suited to medium-heavy and high-consumption systems — particularly SPS-dominant tanks.

7. Warning signs

Alkalinity drops rapidly for no obvious reason — first checkpoint is CO₂ balance. High CO₂ in the water consumes the buffer. Another possibility is precipitation — calcification on equipment or rock surfaces.

Alkalinity won’t stay stable despite dosing — check the dosing schedule. Dosing should be spread evenly through the day rather than delivered in one large shot.

Alkalinity is high but pH stays low — a sign that CO₂ levels are high. The solution is not to add more alkalinity — it is to reduce CO₂ through better ventilation, a CO₂ scrubber or other means.

Summary

Alkalinity is the water’s buffer capacity — its ability to resist pH change. It depletes through calcification and biological acidification reactions and must be actively replenished in all coral-keeping tanks.

The target range is approximately 7.0–9.0 °dKH depending on tank type, but more important than the absolute level is staying within it. Stability protects corals more than any single number imagined to be optimal.

References

1. Peer-reviewed studies

• Allemand, D., Tambutté, É., Zoccola, D. & Tambutté, S. (2011). Coral calcification, cells to reefs. Coral Reefs: An Ecosystem in Transition, 119–150. https://doi.org/10.1007/978-94-007-0114-4_9
• Gattuso, J.-P. et al. (1998). Effect of calcium carbonate saturation of seawater on coral calcification. Global and Planetary Change, 18(1–2), 37–46.
• Marubini, F. & Atkinson, M. J. (1999). Effects of lowered pH and elevated nitrate on coral calcification. Marine Ecology Progress Series, 188, 117–121.

2. Hobbyist literature and brand documentation

• Holmes-Farley, R. (2015). pH and the Reef Aquarium. Reef2Reef / REEFEDITION.
• NYOS Aquatics (2023). ION-B System — Complete Guide. NYOS Aquatics GmbH.
• Fauna Marin GmbH (2025). Alkalinity / Carbonate Hardness — Knowledge Base. https://www.faunamarin.de/en/alkalinity-carbonate-hardness/

3. Books and textbooks

• Borneman, E. H. (2001). Aquarium Corals: Selection, Husbandry, and Natural History. TFH Publications / Microcosm.
• Sprung, J. & Delbeek, J. C. (1994). The Reef Aquarium, Vol. 1. Ricordea Publishing.