WATER TESTING & MAINTENANCE

pH Test Kit

Liquid indicator or probe that measures acidity — and the swing matters more than the value.

Maintaining stage Water

Unplug before servicingHands out of the water firstDrip loop on every cableTest before you dose30 ppm CO₂ ceilingCycle before stocking

Water

Specification
Method	Liquid indicator or electrode
Range	6.0–8.0 typical (wide-range 4.0–10.0)
Accuracy	±0.2 (liquid), ±0.05 (calibrated probe)
Tests per kit	100–400
Read time	Instant (probe), 1 min (liquid)
Storage	Cool, dark, upright

Technical parameters
Target, soft-water tank	6.0–7.0
Target, hard-water tank	7.2–8.0
Daily CO₂ swing	~1.0 pH unit
Dangerous swing	> 1.5 units in 24 h
Test frequency	Weekly, plus after any change
Consistency	Same time of day each test

pH is the logarithm of hydrogen ion concentration, which means every unit is a tenfold change. A tank sitting at pH 6.8 is not "slightly more acidic" than one at 7.8 — it has ten times the hydrogen ion activity, and the biological difference is substantial.

For aquascaping, the number itself is less important than its stability and its daily range. In a CO₂-injected tank, pH predictably falls through the photoperiod as dissolved CO₂ rises, then rises again overnight as the gas is consumed and driven off. A drop of around 1.0 pH unit between lights-on and lights-off is normal and is itself a rough CO₂ indicator. A drop of 2 units, or a value that will not settle, means something is wrong.

Liquid indicator kits are accurate to about 0.2 pH and are the practical choice for most keepers. Probes are faster and can log continuously, but they need two-point calibration and their readings drift as the electrode ages. For a planted tank, a liquid kit read at the same time each day tells you more than a probe read at random moments.

Safety and welfare

pH up and pH down adjusters cause large swings and are a common cause of livestock death. Change pH through substrate, water source and CO₂, not chemicals.
Test at the same time of day every time. A morning reading and an evening reading of the same tank are legitimately different numbers.
Do not store liquid reagent kits where they freeze or in direct sun. Both destroy the indicator.

Using it well

A stable pH of 7.4 is better for most fish than a fluctuating 6.8. Chasing a target number causes more problems than it solves.

If pH will not hold steady, test KH. KH is the buffer, and low KH means pH is at the mercy of every input.

Read the test tube against a white card in daylight, not against a coloured background under a warm bulb.

Test alongside

GH/KH test kitNitrate test kitDrop checkerCO₂ regulator

Note. Parameters are conventional ranges across the category, not the claim of any one manufacturer. Nothing on this page is for sale and no outcome for any plant or animal is guaranteed. Simulator output elsewhere on this site is a simplified model, not a measurement — confirm everything with a test kit and the equipment's own instructions.

pH Test Kit

Using it well

Test alongside

Works alongside