How to Safely Lower or Raise Your Aquarium pH

Chasing the 'perfect' pH causes more dead fish than a slightly wrong number ever will. Here is how to think about pH - and how to change it safely if you really need to.

First: do you actually need to change it?

Most fish adapt happily to a stable pH that isn't their textbook ideal. A steady pH that is slightly 'wrong' is far healthier than a pH that bounces around as you chase the perfect value. The best approach is usually to choose fish that suit your tap water, not to fight your water chemistry.

Understand KH first

Before touching pH, test your KH (carbonate hardness). KH is the buffer that holds pH steady. High KH resists pH change, which is why dumping in pH-down chemicals often does nothing - or causes a dangerous sudden crash later. You generally have to lower KH to lower pH stably.

How to lower pH (gently)

Use natural methods: add driftwood, Indian almond (catappa) leaves or peat, which release tannins and acids; mix in some RO (reverse osmosis) water to dilute the minerals and KH; or run CO2 in a planted tank. Avoid quick chemical 'pH down' products, which give unstable, short-lived results.

Use our water parameter converter to keep track of your readings.

How to raise pH

To raise pH and KH, add crushed coral, aragonite or limestone to the filter or substrate, or use a remineralising product. These dissolve slowly and provide a gentle, stable lift - ideal for hard-water fish like African cichlids and livebearers.

The golden rule

Whatever you do, change pH slowly - no more than about 0.2-0.3 units a day. A fast swing is more dangerous to fish than the wrong value itself.

Frequently Asked Questions