The pH measures the acidity or on the contrary the alkalinity of the water. It is measured using a scale from 0 to 14. The number 7 corresponds to a neutral solution. Values above 7 are basic or alkaline; values below 7 are acidic. The pH reading is logarithmic: a change of 1pH corresponds to a 10-fold increase in acidity or basicity. Let’s take a pH of 5: it is 10 times more acidic than a pH of 6 but 10 times less acidic than a pH of 4. Similarly, a pH of 8 is 10 times more basic than a pH of 7 and 100 times more basic than a pH of 6.

For fishkeeping of course, extremes are meaningless: most fish and plants prefer a pH between 6.0 and 8.0, and only a few species will require more acidic or, on the contrary, more alkaline values. Goldfish are adaptable but prefer a neutral to fairly alkaline pH (7.0 to 8.0 ideally, acidic water is not recommended). The pH is commonly tested using small test kits available in pet shops but a digital pH meter can also be purchased.

Whether you need to regurlarly test the pH will largely depend on the stability (or rather the instability) of the pH in your tank.

If your water is soft or seems to progressively get acidic, or if the pH is unstable, you will need to test your water regularly. The tests should be carried out at the same time of day, preferably in the afternoon, because during the day, as plants produce oxygen and use carbon dioxide, the pH may increase slightly, whereas at night the phenomenon is reversed, as plants and fish consume oxygen and release carbon dioxide, and the pH may then decrease slightly. This simple precaution will prevent misinterpretation. If you suspect that the pH level fluctuates significantly during the day, the pH should be tested twice, in the morning and in the evening, to determine the exact amplitude: an unstable pH level will always require urgent action to be taken. The first measure is to first make a partial change in the water: by replacing the old water with new water, you will add a buffer effect (these are dissolved mineral substances that would have been gradually depleted in your tank and whose property is to provide a certain resistance to pH fluctuations).

For those whose water is hard water, on the other hand, there is little chance of the pH changing: hard water contains large quantities of these dissolved substances that slow down pH changes, and regular water changes will suffice to “recharge” it with buffer substance.

Goldfish are much better than most other species to cope with a rather inadequate pH. However, care should be taken to provide them with water that is compatible with their well-being. A pH that is much too low (usually in a tank that is overcrowded and overloaded with organic waste) can cause acidosis: the symptoms (which are not exclusive, hence the point of testing) are usually apathy and refusal to feed, over-secretion of mucus, fins where blood veins appear. A very high pH can cause, but it is much rarer, an alkalosis – but above all increases the toxicity of ammonia: the fish remains on the surface unable to breathe properly and secrete a lot of mucus. Beyond that, sudden pH variations are the main danger and may put fish under great stress.