pH
The acidity-alkalinity of the water on a log scale. Where it sits matters less than how steady it stays — and what it tells you about CO₂.
What it is
pH is the negative log of the hydrogen ion concentration. The scale is logarithmic — pH 6 has ten times the H⁺ concentration of pH 7, a hundred times pH 8. Most freshwater aquariums sit between 6.0 and 8.0.
Why it matters
A steady pH is far more important than a "correct" one. Most fish in the hobby have been tank-bred for generations and tolerate a wide pH range as long as it doesn't swing. The exceptions: dedicated soft-water breeding tanks (discus, wild Apistogramma), African rift-lake cichlid tanks (need pH ≥ 8), and tanks where pH drops sharply at night because KH is too low to buffer the CO₂ cycle.
For planted tanks running CO₂ injection, pH is the live readout of how much CO₂ is dissolved. A 1.0 pH drop from non-injected to lights-on usually means CO₂ is in the target band.
How to test
A calibrated pH meter is worth the cost if you run CO₂ or breed soft-water species. Otherwise a liquid drop test (JBL pH, Tetra pH) or a strip is fine for monthly checks. Test at the same time of day every time — pH cycles by 0.5–1.0 over a CO₂-injected day.
What high and low look like
Below 5.5: most fish stressed; acid burn on gill membranes within days. Between 5.5 and 6.5: planted-tank territory, fine for South American and Southeast Asian species. Between 6.5 and 7.5: middle ground, suits most community fish. Above 8.0: alkaline territory, suits African cichlids and brackish setups but stresses soft-water species.
How to fix
Too high — CO₂ injection lowers pH naturally; for non-planted tanks, peat in the filter or dilution with RO water. Too low — raise KH with potassium bicarbonate; pH follows. The CO₂ calculator converts pH and KH into a CO₂ number so you can dial in the injection rate against the target band.
