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pH in the reef aquarium — what it is and why it fluctuates

Basics Published: 3 June 2026

pH is one of those parameters measured often but understood rarely. This article covers what pH means, why it changes between day and night, what it means for corals — and what to do if it is too low.

What pH measures

pH tells us how acidic or basic the water is. More precisely: it measures the concentration of free hydrogen ions (H⁺) in the water. More hydrogen ions means more acidic — and lower pH.

The scale is logarithmic, which means in practice that even small changes are biologically significant. pH 7.9 is 26% more acidic than pH 8.0. pH 7.8 is nearly 60% more acidic than pH 8.0. This does not mean the tank collapses the moment pH drops by a decimal unit — but it does mean readings should not be interpreted linearly.

In natural seawater, pH is typically 8.0–8.3. Reef tanks aim for the same range.

Why pH fluctuates between day and night

This is one of the most commonly confusing phenomena for new hobbyists: pH rises during the day and falls at night, every day, completely normally. The reason is biological.

During the day: Corals and algae photosynthesise. Photosynthesis consumes CO₂ from the water. As CO₂ concentration drops, pH rises — the water becomes slightly more basic.

At night: Photosynthesis stops. All tank inhabitants — corals, fish, bacteria — respire and produce CO₂. CO₂ concentration rises, pH falls.

Typical diel variation is 0.1–0.3 pH units. The variation is natural and not inherently harmful, as long as pH stays within a safe range at both points in the daily cycle.

Safe range and target values

StatuspH valueMeaning
Too low< 7.8Calcium carbonate begins to dissolve — coral skeletons at risk
Acceptable lower limit7.8Calcification slows, no acute danger
Target range8.0–8.3Normal reef aquarium
Optimal8.2–8.4Calcification at maximum efficiency
Too high> 8.5Precipitation risk increases, not normally a problem

The single most important rule: the night-time minimum must not fall below 7.8. The daytime maximum is rarely a problem in a normal tank.

What affects pH most

Two factors govern pH almost entirely:

1. CO₂ concentration in the water CO₂ dissolved in water forms carbonic acid, which releases H⁺ ions and lowers pH. The more CO₂ in the water, the lower the pH. CO₂ concentration is affected by biological processes (respiration vs. photosynthesis) and the CO₂ level of indoor air.

2. Alkalinity Alkalinity is the water’s buffer capacity — its ability to resist changes in pH. High alkalinity means pH does not drop as much even when CO₂ is produced. Low alkalinity means pH swings more readily.

These two factors work together: the same CO₂ load lowers pH more at low alkalinity than at high alkalinity.

Indoor CO₂ — a particular problem in Finland

Outdoor air CO₂ concentration is approximately 420 ppm. Indoors, where people and pets breathe in an enclosed space, concentration easily rises to 800–1500 ppm — especially in winter in a tightly sealed Finnish building.

The protein skimmer draws air directly from indoors and pumps CO₂-rich air into the water. If indoor CO₂ is twice that of outdoor air, the skimmer is actively lowering the tank’s pH continuously — more than biological respiration.

In Finland during winter, this is one of the most common causes of low pH, and it often goes unidentified.

How pH is measured

An electronic pH electrode is the only way to monitor pH continuously. The electrode is connected to a tank controller or separate meter, and pH is recorded in real time as a graph. This is important because a single measurement tells you nothing about the diel cycle — you need to compare morning and afternoon readings.

The electrode must be calibrated regularly — typically once a month — using pH 7.0 and 10.0 calibration solutions.

Test kits are inaccurate for pH measurement. They can give a rough indication for a single check, but tracking the diel cycle or detecting small changes is not possible with a test kit. An electrode is practically essential for pH monitoring.

What to do if pH is too low

The most common cause is indoor CO₂. The easiest diagnostic test: take a cup of tank water and aerate it with outdoor air (open a window, use a airstone) for 30–60 minutes. If pH rises significantly, the problem is indoor CO₂ — not tank chemistry.

Simplest fix: Open a window near the skimmer daily for a few hours. In winter this is not always possible — the skimmer’s air intake tube can then be routed outside through the wall.

Other strategies and comparisons are covered in the practice article.

What a drop in pH means for corals

Corals build their skeletons from calcium carbonate. This process — calcification — is energy-consuming, and it slows when surrounding pH drops. The reason is straightforward: lower pH means the coral’s cells must work harder to maintain the chemical conditions at the skeleton’s growth surface.

As pH drops below 7.7–7.8, calcium carbonate begins to dissolve — the coral’s skeleton slowly breaks down in acidic water.

Practical meaning: corals in a tank at pH 8.0–8.3 grow and thrive. pH 7.8–8.0 is acceptable but slows growth. Below 7.8 is a problem that needs to be addressed.

Summary

References

1. Peer-reviewed studies

2. Hobby literature and brand documentation

3. Books and textbooks