pH
The pH indicates how acidic or alkaline the water is on a scale from 0 to 14. For aquariums the ideal value depends on the type of aquarium and the species hosted.
Detailed explanation
pH measures the concentration of hydrogen ions in water. A pH of 7 is neutral. Below 7 lthe water is acidic. Above 7 it is alkaline. The scale is logarithmic: a pH of 6 is ten times more acidic than a pH of 7.
For the reef, stability matters more than absolute value. A constant pH of 8.0 is better than one that fluctuates between 7.8 and 8.4 throughout the day. The oscillations stress the corals much more than a value slightly out of range.
In tropical freshwater aquariums the pH depends on the species. Discus like pH around 6.0-6.5. Malawi African cichlids prefer 7.8-8.6. There is no universal value: it all depends on what you host.
In ponds the issue becomes more complicated because the photosynthesis of the algae causes the pH to rise during the day while respiration lowers it at night. With good KH these oscillations remain contained. Without adequate buffer the pH can jump by 2 points within 12 hours.
For brackish aquariums, a pH between 7.5 and 8.3 is needed which reflects the conditions of natural estuaries.
Reference values by aquarium type
| Aquarium type | Recommended range |
|---|---|
| Marine (reef) | 7.8 - 8.4 |
| Tropical freshwater | 6 - 7.5Depends on the species. African cichlids: 7.8-8.6 |
| Coldwater / pond | 7 - 8.5Goldfish and koi tolerate a wide range but suffer from rapid oscillations |
| Brackish | 7.5 - 8.3 |
How it is measured
It can be measured with litmus paper (not very precise), drop tests (good precision) or electronic probes (maximum precision but require regular calibration). For marine aquariums a probe is almost essential because continuous monitoring is required. In freshwater aquariums a weekly drop test is usually sufficient.
What happens when it is too low
The fish are breathing heavily. The corals close. The gills become irritated by producing excess mucus. A sudden drop below 7.5 in marine aquariums can be fatal within hours. In freshwater aquariums a pH below 5.5 inhibits the nitrogen cycle with accumulation of ammonia.
What happens when it is too high
Above 8.5 there is a risk of calcium precipitation in marine aquariums. In freshwater aquariums, too high a pH damages the mucous membranes of Amazonian fish. Aquatic plants suffer because they are unable to absorb dissolved CO2.
Coralia Lab monitors this parameter with AI diagnostics. Tank Score alerts you when a value moves outside the recommended range for your aquarium.
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