What pH Measures
pH measures the concentration of hydrogen ions in water on a scale of 0 to 14, with 7 as neutral, below 7 as acidic, and above 7 as alkaline. Because the scale is logarithmic, each whole-number step represents a tenfold change in hydrogen ion concentration — meaning a pH of 6 is ten times more acidic than a pH of 7, not just slightly more.
Why pH Matters for Fish
Different species have evolved for different natural water chemistry — soft, acidic blackwater rivers for fish like Discus and Tetras, versus hard, alkaline rift lakes for many African cichlids. Beyond species preference, pH has an outsized effect on the toxicity of other compounds in the tank.
The Ammonia Connection
The clearest example is ammonia: higher pH shifts a larger share of total ammonia into its toxic un-ionized (NH3) form, meaning the same ammonia reading can be relatively harmless at a low pH and dangerous at a high one. This is why pH is always read alongside ammonia when assessing real risk to fish.
Stability Over a 'Perfect' Number
Most community fish tolerate a pH outside their textbook-ideal range reasonably well, as long as it stays stable and they were acclimated to it gradually. Sudden swings — even within a range that looks acceptable on paper — tend to stress fish far more than a static, slightly-off value. Chasing an exact number with chemical additives is a common beginner mistake that often introduces the very instability it's meant to prevent.
What Causes a pH Crash
A pH crash occurs when carbonate hardness (KH), the mineral buffer that resists pH change, gets depleted. Very soft source water, acidifying driftwood or substrates, and CO2 buildup from overstocking or poor surface agitation can all deplete this buffer, letting pH fall rapidly and severely.
Fixing a Crash the Right Way
The fix is to rebuild KH gradually — using a carbonate buffer product or crushed coral/aragonite — rather than treating pH directly. Since KH and pH are linked, correcting pH alone without addressing the underlying buffer capacity usually causes the crash to recur.
CO2 Injection and pH
In planted tanks, injected CO2 dissolves into carbonic acid and lowers pH, which is expected and often a useful indicator that CO2 levels are adequate for plant growth. The risk is injecting CO2 faster than KH can buffer it, causing sharp pH swings between lights-on and lights-off. A drop checker or CO2 controller helps keep this in a safe range.