pH
pH
What is it
pH measures the concentration of hydrogen ions in water on a logarithmic scale from 0 to 14. A pH of 7.0 is neutral; lower values are acidic and higher values are alkaline. Because the scale is logarithmic, pH 6.0 is ten times more acidic than pH 7.0.
In nature, Amazonian rivers reach pH 4.5-6.5 because of tannins and humic acids. The African Rift lakes of Malawi and Tanganyika reach pH 8.0-9.0 because of limestone geology. Most commercially available tropical freshwater fish have adapted to pH 6.5-7.5.
pH in an aquarium is never static: injected CO2 lowers it during the day, respiration raises it at night, and water changes and substrate ageing also alter it.
Why it matters
pH directly changes ammonia toxicity: at pH 7.0, only 1% of total ammonium is in the toxic NH3 form, while at pH 8.0 the share rises to 10%. An alkaline aquarium therefore tolerates less organic load.
For plants, pH determines nutrient availability. Iron becomes insoluble above pH 7.5, causing chlorosis even in well-fertilized tanks. Dissolved CO2 is readily available for photosynthesis below pH 7.0; above that, more of it becomes bicarbonate, which is less usable.
For invertebrates, extreme pH damages chitin and compromises moulting. Crystal Red Caridina prefer pH 5.8-6.5 with active soil. Most aquarium animals can adapt to a suitable range, but sudden shifts impair gill function and bacterial performance.
Ideal ranges
Interactions with other parameters
KH buffers pH, and dissolved CO2 lowers it. Ammonia is mostly the less harmful ammonium form below pH 7.0, whereas above pH 7.5 even a small trace of ammonia can become lethal. Acidic pH also keeps iron in a more bioavailable state for plants. Always read pH together with KH and temperature.
| Tank type | Min | Max | Unit |
|---|---|---|---|
| Tropical community | 6.5 | 7.5 | pH |
| Planted high-tech | 6.0 | 6.8 | pH |
| Planted low-tech | 6.5 | 7.8 | pH |
| Shrimp tank | 6.0 | 7.0 | pH |
Out of range: what happens
pH that is too low (below 5.5) can corrode gills, cause osmotic stress, slow the nitrogen cycle because nitrifying bacteria work poorly below pH 6.0, and trigger a potentially fatal acid crash. It also stresses hard-water species.
pH that is too high (above 8.0) makes ammonia much more toxic, precipitates iron and micronutrients so plants cannot use them, and can irritate the mucosa of soft-water fish.
Rapid swings (more than 0.5 points within a few hours) cause osmotic shock and damaged mucosa; fish may rub against decorations and mortality can follow in subsequent days. Rapid correction can also shock shrimp.
Common Myths
- A pH of 7.0 is always ideal; many species, including cardinal tetras and discus, suffer at 7.0 and prefer pH 6.0. Stable water within a suitable range is usually safer than chasing an exact number.
- pH changes water hardness; pH is acidity, while GH and KH are hardness measures. They are related but distinct, and pH-down products do not fix the cause if KH remains unchanged.
Measurement and adjustment
How to measure
A liquid reagent test such as API or JBL is the most common method: add drops to a water sample and compare the colour, with about ±0.2 unit precision.
An electronic pH probe provides continuous readings with ±0.01 precision, but needs regular calibration with pH 4.0 and 7.0 buffer solutions. It is ideal for tanks with pressurized CO2.
Multi-parameter test strips are useful for quick screening but have limited precision (±0.5); do not rely on them alone for important decisions.
Always measure at the same time of day, because pH changes through the photoperiod, especially with injected CO2.
How to adjust
Change pH by changing its buffering system, not by chasing numbers with acids or bases. To lower it, use CO2 injection, which is the most controllable and plant-friendly method; active zeolite- or allophane-based soil that lowers and buffers pH to 6.0-6.5; peat in the filter; catappa leaves or wood that release tannins; or RO water to dilute buffers.
To raise pH, use calcareous rocks or crushed coral in the filter, commercial bicarbonate buffers, or more aeration to drive off excess CO2.
For stability, keep adequate KH (at least 2-3 dKH) as a buffer. Active soils consume KH over time, so monitor both values together. pH-down products do not address the cause: without changing KH, the value often rebounds.
Pro Tips
- Measure pH before the lights and CO2 come on, and again before they switch off: the difference gives an empirical estimate of daily CO2 use. For CO2 tanks, compare the degassed reading with the peak-injection reading; a 1.0 pH drop targets about 30 ppm CO2.
- Acclimate sensitive shrimp slowly when moving between different pH and KH values.