The short answer
Guppies do best at a pH of about 7.0 to 8.0 — neutral to slightly alkaline — in moderately hard to hard water. They’re classic hard-water livebearers, so unlike soft-water tetras they actually prefer some minerals and a higher pH. That said, don’t obsess over hitting an exact figure: guppies are famously adaptable, and a stable pH they’ve acclimated to matters far more than chasing a “perfect” number. If your tap water sits in this range, you’re already set.
Why guppies like it hard and alkaline
Guppies evolved in mineral-rich waters and use calcium and magnesium for health, colour and breeding. Higher GH supports them directly, and the KH that usually comes with hard water keeps pH stable and alkaline — exactly what they want. This is why guppies often struggle in soft, acidic blackwater setups built for other fish. For the hardness side, see KH and GH explained.
Don’t chase the number
The most common guppy mistake is fiddling with pH chemicals to hit 7.5 on the dot. Every adjustment risks a swing, and swings stress fish more than a steady “off” value ever will. Test your tap water with a liquid test kit — if it lands anywhere from 7.0 to 8.0, leave it alone and keep it stable with regular water changes.
If you need to adjust
To nudge water more guppy-friendly, see making water harder. For general control, see raising or lowering pH, and browse guppy care in our guppy guide. More in the water testing hub.