pH
pH is the parameter people chase hardest and the one where chasing does the most damage. Each unit is a tenfold change in hydrogen ion activity, so pH 6 is ten times more acidic than pH 7. Most aquarium fish available today are captive-bred and adapt to a wide range; almost none tolerate a pH that swings.
In one sentence
pH measures the activity of hydrogen ions on a logarithmic scale. In freshwater aquariums it matters mostly because it decides how toxic ammonia is, and because most fish tolerate a stable wrong pH better than an unstable right one.
How to read a measurement
Bands are guidance for interpreting a reading, not thresholds with sharp edges. Sensitivity varies by species and by how long the exposure lasts.
| Band | Range | Verdict | What it means |
|---|---|---|---|
| Strongly acidic | ≤ 5.5 | watch | Blackwater conditions. Biological filtration slows markedly below about pH 6. |
| Acidic | 5.5–6.8 | acceptable | Suits soft-water South American and South-east Asian species. |
| Neutral | 6.8–7.4 | target | Suits the widest range of commonly kept species. |
| Alkaline | 7.4–8.2 | acceptable | Suits livebearers and many African species. Ammonia toxicity rises here. |
| Strongly alkaline | ≥ 8.2 | watch | Rift lake conditions. Any ammonia present is substantially more dangerous. |
Why stability beats the number
A fish moved between pH 7.8 and 7.2 twice a day is under continuous osmoregulatory stress. A fish held steadily at 7.8 when its care sheet says 6.5 is usually fine, particularly if it is captive-bred. This is the single most useful thing to know about aquarium pH, and it is the opposite of what most pH-adjusting products imply.
Why pH moves
- CO₂. Dissolved carbon dioxide forms carbonic acid, so pH falls at night when plants respire and rises during the day when they photosynthesise. A daily swing of 0.2–0.4 in a planted tank is normal.
- Nitrification. The nitrogen cycle produces hydrogen ions and consumes carbonate hardness, pushing pH down over weeks.
- Degassing. Tap water is often supersaturated with CO₂ and its pH rises over 24 hours in a bucket. Test tap water after standing, not straight from the tap.
- Substrate and hardscape. Aquasoils buffer downward; limestone, coral sand and shells buffer upward.
Changing pH the right way and the wrong way
The wrong way is a bottled 'pH down' product added to water with high carbonate hardness. The buffer consumes it, the pH returns, and repeated dosing produces exactly the instability that harms fish.
The right way is to change what determines pH in the first place: dilute with reverse-osmosis water to reduce the buffer, add botanicals and peat for a gentle downward influence, or add carbonate materials for an upward one — and then let it settle.
How it is measured
- Liquid test kit
- Digital pH meter (needs regular calibration)
- Test strips
Typical aquarium range
- Typical
- 6–8 pH
Common misconceptions
| Often said | What is actually the case |
|---|---|
| pH 7.0 is neutral, so it is best for fish. | Neutral is a chemical definition, not a biological target — and the neutral point itself shifts slightly with temperature. Plenty of healthy aquarium water sits well away from 7.0. |
| You can lower pH with driftwood and it will stay there. | Tannins from wood and leaves push pH down only if the carbonate hardness is low enough to allow it. In hard tap water, wood stains the water and the pH barely moves. |