The short answer
Aquarium pH drifts down because acids constantly build up — from fish waste, decaying matter, driftwood tannins and dissolved CO2 — and something has to neutralise them. That something is KH (carbonate hardness). If your KH is low, there’s no buffer to absorb those acids, so pH slides down over days or weeks. The fix isn’t to fight the pH directly but to restore the KH buffer so it holds steady. A dropping pH is nearly always a low-KH story.
The real culprit: low KH
Think of KH as a sponge soaking up acid. In a well-buffered tank, pH barely moves between water changes. In a low-KH tank, that sponge is empty, so every bit of acid from waste and CO2 pulls pH straight down — sometimes fast enough to crash. Test your KH with a liquid test kit: if it’s very low, you’ve found your answer.
Where the acids come from
- Biological waste — the nitrogen cycle produces acids as it processes ammonia.
- CO2 — from fish respiration or injection, forming mild carbonic acid.
- Driftwood, leaves and peat — release tannins that lower pH (see removing tannins).
- Skipped water changes — let acids accumulate unchecked.
How to stabilise it
Rebuild the buffer rather than chasing the number. Add crushed coral to the filter or substrate for a slow, self-limiting KH boost, and keep up regular water changes to refresh KH and export waste.
If pH has already fallen sharply, see how to fix a pH crash and what a buffer is. More in the water testing hub.