Next to the temperature, every species profile lists two or three more numbers: pH, dGH and sometimes dKH. They are not a formality — more often than not they are what stands behind the unexplained death of a fish that "was fed properly". Here is what each of them means and what to do about it.

pH — acidity

pH tells you whether the water is acidic or alkaline. The scale runs from 0 to 14, with 7 in the neutral middle: below that the water is acidic, above it alkaline.

The one thing worth remembering is that the scale is logarithmic. The difference between pH 7 and pH 6 is not "one unit": the water becomes ten times more acidic. Between 7 and 5 it is a hundred times. Shifting the pH "by just a point and a half" in one evening is therefore not a small correction but an abrupt change of environment.

Different fish need different water. Inhabitants of Amazonian blackwater and the peat swamps of South-East Asia — discus, apistogramma, licorice gouramis, many tetras — live at pH 4.0–6.5. Cichlids of the great African lakes, livebearers and most brackish-water species need the opposite: alkaline water at 7.5–8.5.

dGH — general hardness

dGH is the amount of dissolved calcium and magnesium salts. Soft water (up to 6 dGH) is typical of rainforests; hard water (15 dGH and above) of lakes lying on limestone.

Hardness matters more than it seems: osmotic pressure depends on it, and with it the work of the kidneys and gills. A soft-water fish in hard water constantly fights an excess of salts, while a hard-water fish in soft water loses electrolytes. That is where the listlessness comes from, and the "causeless" death a few weeks after purchase.

dKH — carbonate hardness

dKH is the content of carbonates and bicarbonates. On its own it means little to the fish. But it is dKH that keeps the pH steady: carbonates act as a buffer and damp the acidification that goes on in every aquarium as fish breathe and organics decay.

A low dKH is the main cause of sudden pH crashes. Below about 3 there is practically no buffer left, and the pH can fall by a whole unit overnight. So when you prepare soft, acidic water, keep the dKH at least around 2–3, and better still watch it with a separate test.

How to measure

Liquid test kits are cheaper and more accurate than strips: strips scatter by half a pH unit and deteriorate quickly in a damp cupboard. An electronic pH meter is convenient, but it has to be calibrated against buffer solutions: without that it lies worse than the strips.

It is worth testing three things: the aquarium water, tap water after standing, and the water you use for changes. The difference between the last two is the most common source of stress during water changes.

Lowering pH and hardness

  • Reverse osmosis. RO water is almost free of salts and is blended with tap water in the proportion you need. The most predictable method.
  • Peat and alder cones in the filter or in a separate container: their tannins gently acidify the water and give it a tea-coloured tint.
  • Driftwood and leaf litter work the same way, only more slowly. See the article Which tree leaves can be used in an aquarium.

Bottled "pH down" products are close to useless without RO water. With a high dKH the buffer pulls the pH back within a day, and the fish gets a swing instead of stability.

Raising pH and hardness

  • Carbonate substrate: coral sand, limestone or crushed shell in the filter or the substrate.
  • Purpose-made salts for African cichlids and for brackish aquariums — they raise both the hardness and the pH.
  • Baking soda raises only the carbonate hardness and the pH; it adds no calcium or magnesium, so it will not do on its own.

Speed matters more than precision

A healthy fish tolerates an unfamiliar pH far better than a rapid change of it. A safe pace is no more than 0.3 pH units per day and about 2 dGH per day.

Hence a simple rule: if your aquarium water differs greatly from what you want, get there through water changes over a week or two, not in one go. The same goes for new fish: slow acclimatisation, half an hour to an hour, is there precisely so that a difference in pH and hardness does not land as a blow.

When to leave the parameters alone

If the fish is alive, eating and showing no signs of illness while the parameters are only slightly outside the recommended range, it is wiser to leave everything as it is. Stable water of the "wrong" hardness is safer than water that is perfect on paper but swinging. There are three cases worth acting on: the parameters are far from what is needed, you are conditioning fish for spawning, or you keep a species that must have soft acidic water.