The three numbers that decide whether your fish thrive — how to test, read and gently adjust them.
By Lin Wei · 2026-09-02
pH is a logarithmic scale of acidity: 7 is neutral, below is acidic, above is alkaline. Because the scale is logarithmic, a shift from 7.0 to 6.0 is a tenfold increase in acidity — this is why sudden pH crashes kill, and why 'slow and stable' is the only safe adjustment strategy.
GH (general hardness) measures calcium and magnesium — the minerals fish absorb through their gills and that snails and shrimp need to build shells. Chinese soft-water mountain species do best at 5–12 dGH; livebearers and many African species prefer 12–25.
KH (carbonate hardness) measures the water's buffering capacity — its ability to resist pH change. KH is the reason a tank's pH stays put day after day, or slides without warning. A KH of 3–8 dKH keeps most community tanks stable; below 2, pH can crash overnight.
Fill a glass with tap water, let it sit 24 hours (or aerate it) so dissolved CO2 equalises and the reading settles, then test pH, GH and KH. This trio is your baseline, and for 90% of species on this site it is already suitable: most Chinese natives come from moderately soft, neutral water.
Then test your tank. If the tank's pH drifts from the tap reading over a week, your KH is being consumed — usually by acidic waste, driftwood, or CO2 from plants at night. Raising KH slightly with a little crushed coral in the filter fixes the drift far more safely than chasing pH directly.
Fish do not experience a number; they experience change. A paradise fish living its whole life at pH 7.6 is healthier than one moved between 6.8 and 7.4 every week. Before any adjustment, ask whether the fish in front of you actually needs it — the species pages list each one's range.
When adjustment is genuinely needed, change at most 0.2 pH units per day. Natural methods move slowly and forgive overdosing; chemical pH products swing fast and crash hard, which is why experienced keepers avoid them for routine use.
To soften and gently acidify: driftwood, catappa leaves, and peat in the filter — ideal for supporting wild-caught soft-water species. To raise hardness and buffer: crushed coral or aragonite in a media bag, dosed small and increased gradually.
For precision work (breeding discs or sensitive species), mix tap water with RO water to hit a target GH, then remineralise with a commercial buffer to restore KH. Never use 'pH up' or 'pH down' products in a stocked tank except as a temporary emergency measure.
Between 6.5 and 7.5, but the honest answer is: the pH of your tap water, if it sits in that range. Most commonly kept species — including nearly all the Chinese natives on this site — adapt perfectly to a stable mid-range value, and chasing a species-specific ideal causes more harm than the number itself prevents.
Place a small amount of crushed coral or aragonite in the filter or substrate and test weekly. KH will climb slowly over one to two weeks; add more only if needed. This simultaneously buffers pH upward and prevents the overnight crashes that low-KH tanks suffer.
Both, for different reasons. GH matters to osmoregulation, shell-building snails and shrimp, and the general vitality of the fish. KH matters to stability — it keeps pH from swinging. For fish health directly, GH first; for avoiding disasters, KH first.
A KH that had been silently consumed finally hit zero, removing the buffer and letting accumulated acids swing pH down fast — often after a big water change with soft source water, or in tanks with lots of driftwood and leaf litter. Remineralise with crushed coral and raise KH gradually over days; an overnight crash from, say, pH 7 to 5.5 is a genuine emergency requiring immediate partial water changes with buffered water.
Yes — and this is why the weekly water change is the single most powerful maintenance act. It dilutes nitrate (the end product of the cycle) and replaces acids and consumed minerals simultaneously, restoring both pH stability and GH/KH toward your tap-water baseline.
Yes: blending RO or collected rainwater with tap water in known ratios is the cleanest way to reach a target GH for soft-water species. Remineralise the blend so KH stays above 3, match temperature, and change only a portion of the tank water at a time so the shift is gradual.