pH and KH

What it is

pH describes how acidic or basic the water is on a logarithmic scale, which means each whole step is a tenfold change rather than a small one. KH — carbonate hardness, usually reported in degrees or in ppm as calcium carbonate — describes how much buffering capacity the water has to resist that change. They are best understood as a pair. pH is the reading; KH is the reason the reading holds still. Water with generous KH shrugs off the acids that biological processes produce all day long. Water with almost no KH does not, and its pH can move a long way overnight.

Why it matters

Almost every pH problem in a home aquarium is really a KH problem. A tank whose pH drifts down week after week, or swings between one test and the next, has usually run its buffer down. Chasing the pH number directly — with pH-up and pH-down products — treats the symptom and often makes the swings worse, because you are moving a number that has nothing holding it. The other half of the picture is that fish care far more about stability than about hitting a particular value. A steady 7.8 is kinder than a 7.0 that visits 6.4 every Tuesday, and the right target depends on the species you keep, which is why we point you at the requirements for your fish rather than printing one number for everyone.

What each band means

How to test it

1. Test at the same time of day, every time. In a planted tank pH is measurably higher in the evening than at dawn, and comparing the two looks like a swing that is not one.
2. Test KH in the same session. A pH number without a KH number cannot be interpreted.
3. Use the high-range pH bottle as well if your kit has one, and use it whenever the normal-range test reads at the very top of its card. The standard test tops out and quietly stops responding.
4. Let a tap water sample sit uncovered for a day before testing it. Tap water is often supersaturated with dissolved gas, and its pH out of the faucet is not its pH once it has equilibrated in your tank.

How to move it

1. Fix KH first, and mostly by changing water regularly with water that has some buffer in it. Consistent water changes are the ordinary way tanks keep their pH steady.
2. Look at what is in the tank. Driftwood and peat push pH and KH down over months; crushed coral, limestone and shell push them up. Substrate and hardscape do more slowly and steadily than any bottle does quickly.
3. Check whether the reading is real. A pH that drops only in the morning is the day-night carbon dioxide rhythm and not a fault.
4. Treat pH adjusters as a last resort with an expiry date. They move a number without adding what holds it, so the tank comes back to where it was and takes the fish through a round trip on the way.
5. Change any pH deliberately and slowly. Species-appropriate targets vary widely, so use the requirement for the fish you actually keep rather than a general figure.

Where the reading goes wrong

• Reading the normal-range pH test at the top of the card and assuming that is the value. Above its ceiling the test simply stays the same color.
• Testing a sample that has been sitting in a warm room in an open tube for an hour. Gas exchange has already moved it.
• Assuming a stable pH means a healthy buffer. A tank can read the same for weeks and then fall quickly once KH finally runs out.
• Adjusting pH between tests and testing again immediately. Buffered water resists, then moves later, which produces overcorrection.

Problems that start here

• Act this week

  My pH reads differently every time I test it

• Act this week

  My test strips and my liquid kit give different answers

• Act today

  My new tank is reading ammonia and there are already fish in it

• Act this week

  Black or dark red tufts are growing on my plants and decor