Roger's Aquatics Pages

pH

What is pH? It is a measure of the acidity or alkalinity of something: in the case of fish keepers, the water in the tank. The abbreviation pH comes from the German for Power of the Hydrogen (ion). It is actually a measure of the concentration of Hydrogen ions in the water.

It is defined as -1(log10(X)) where X is the concentration of Hydrogen ions in Moles per Litre. A mole is the number of atoms in 1 gram of Hydrogen – so X could equally be expressed as the number of grams of hydrogen ions in a litre. It just so happens that pure water is pH7, and "by chance" the scale runs from 0-14. Actually it isn’t really by chance it was actually a bit of smart thinking by someone.

pH1 is very acid, concentrated Hydrochloric acid, or other strong acid. pH14 is very alkaline, such a concentrated Sodium Hydroxide (Caustic Soda) or similar. Fresh water is generally in the range pH5 – pH9, although they are extremes and most fish are happy in the range pH6 – pH8. There are some exceptions, of course. The water in the Amazon can get lower than pH5, and fish such as Discus can live in and appear to enjoy it, and at the other extreme the Soda Lakes of Africa can be much more alkaline than pH9 – they are alkaline enough to take the skin off your leg. Wonder of wonders - and there are fish that live there too. Curiously they are also Cichlids! - Oreochromis alcalicus the Soda Tilapia, a very strange beast that lives in hot soda springs pH10 and over 40C

Naturally water with a low pH is soft, and water with a high pH is hard. This often is not the case with Tap Water where the ground water is soft and acid, the local water supplier will often increase the pH by adding Caustic Soda, which leaves the water soft. This is to prevent the water leaching toxic heavy metals from pipes.

Hardness is often caused by bicarbonates. This makes the water more pH stable because the bicarbonates act as a buffer. Nitrification creates Nitric acid which reacts with the bicarbonates to form Nitrates. Nitric acid is a strong acid, when all the buffering agents in the water are used up, the pH can drop dramatically. This is known as a pH Crash. It is common in soft acid waters used for keeping Discus and similar fish, but is less common in hard water.