pH
A 0-14 scale measuring how acidic or basic water is; 7 is neutral. Most freshwater aquarium fish tolerate 6.5-8.0. Stability matters more than hitting a specific target.
pH is a logarithmic measure of hydrogen-ion concentration. Each whole number represents a 10× change — pH 6 is 10× more acidic than pH 7, and 100× more acidic than pH 8.
In aquariums, pH is influenced by: source-water chemistry (tap water varies by region), substrate (some sands and gravels are inert; crushed coral raises pH), driftwood and peat (lower pH via tannins), CO₂ levels in planted tanks (higher CO₂ = lower pH), and biological activity (decomposition produces acids).
For most beginners, the most important rule is: pH stability beats pH targeting. A tank that's stably at 7.8 is healthier than one that's whipsawed between 6.8 and 7.4 trying to chase a 'better' number. If your tap water is pH 7.8 and your fish tolerate 6.5-8.0, just match tap and don't fight it.
Beginner-safe rule
Conservative beginner rule: test your tap-water pH and your tank pH. If they're within 0.3 of each other and within the target species' tolerance range, leave it alone. Sudden swings of >0.5 pH stress fish; gradual drift across a week is usually fine.