Understanding and Managing pH

What pH is, what drives it in an aquarium, how carbonate hardness buffers it, the danger of a pH crash, and how to adjust pH safely.

What pH is

pH is a logarithmic measure of the hydrogen ion activity in water, defined as the negative base-10 logarithm of hydrogen ion activity. On the 0-14 scale, 7 is neutral, values below 7 are acidic, and values above 7 are alkaline. Each whole unit represents a tenfold change in hydrogen ion concentration, so small numbers represent large chemical differences.

What drives pH

In an aquarium, pH is influenced by dissolved carbon dioxide, the carbonate buffering system, biological acids from waste breakdown, and the mineral content of the source water. Carbon dioxide from respiration tends to lower pH, while carbonates resist that change.

How buffering works

Carbonate hardness (KH), the alkalinity of the water, is the buffer. Bicarbonate and carbonate ions neutralise added acid, keeping pH stable. The more carbonate alkalinity present, the more acid the water can absorb before pH moves.

The pH crash

When alkalinity is depleted, the buffer fails and even a small addition of acid causes a sharp pH drop, known as a pH crash. Biological acidification in a tank with very low KH can exhaust the buffer over time, so maintaining adequate KH prevents sudden crashes.

Adjusting pH safely

Because each pH unit is a tenfold change, adjustments must be gradual to avoid shocking fish. Stable, slightly off-ideal pH is generally safer than a fluctuating one. The most durable way to change pH is to change the underlying chemistry (KH and CO2) rather than chasing the reading with quick additives.

Common mistakes

Using acid or alkali products to force a target value without addressing KH leads to instability and rebound. Mixing source waters of very different chemistry, or large rapid changes, can swing pH faster than fish can tolerate.