The pH value tells us about acidic and alkaline processes that take place in liquids. It is a very important information for the keeping of an aquarium, as variations in this value can bring drastic changes to the environment of our animals. Acidic substances reduce the pH value and alkaline ones increase it. Measuring is done on a scale that goes from 0 (acidic) to 14 (basic). The value at the middle of the scale, 7.0, indicates neutrality between acidic and basic. If, for example, today we measure a pH value of 7.5 in our aquarium, while yesterday we measured 8.5, it means that the water in our aquarium is ten times more acidic than it was before. In this case, scientists talk about “logarithmic negative decay”


What modifies the pH value?

Every acidic substance that ends up in our aquarium, or that develops in it, reduces the value of pH. In the same way, every alkaline substance increases it. Even in a reef aquarium that works harmoniously the pH value changes over time. Normally, it is especially low in the morning and it increases during the day, to then decrease again during the night. These changes happen mainly because of differences in the content of carbon dioxide in the water. Animal breathing and plant activity, algae in a marine aquarium for example, generate CO2, reducing the pH. The photosynthesis that happens during the day consumes large quantities of CO2, causing the acid content in the water to decrease, and the pH value to increase. Towards the end of daylight hours, values of 8.4 or 8.5 are normal. At night, however, the plants stop absorbing carbon dioxide, and the CO2 that is created stays in the water. And now, the crucial question: how much alkaline substance, which tends to bond with CO2 and neutralize it, or absorb it, is there in the water? In this regard, we must take in consideration carbonates, which we can measure by their carbonate hardness. With high carbonate hardness, which means relatively hard water, for example at 7,0° dKH, our aquarium will be able to absorb CO2 until the next morning, without decreasing in pH and entering the danger zone below 7.8. However, if our carbonate hardness is around values of 1° or 2° dKH (very soft water), it means that we do not have a large alkaline reserve available to work as a buffer, and we risk that the pH value will collapse to low levels during the night. Especially if we consider that even during bacterial activity for the transformation of organic matter, for example of leftover food or the excrement of the aquarium’s inhabitants, acidic metabolism products are created, which influence the acidity of the water, it becomes clear how important it is to have an adequate reserve of alkaline buffer. Stabilising the pH value In order to stabilise the pH value in an aquarium, as in reducing daily fluctuations, we have to maintain a sufficiently high reserve of alkaline buffer. This can be obtained with, for example, a partial substitution of water done regularly, about ten percent every month. Another possibility is adding an appropriate buffer of carbonate hardness. In this way, we provide alkaline substances that neutralise the acidic ones, stabilising the pH value. If we replace the water that has evaporated with tap water, we still add carbonates, and of course a calcium reactor can help increasing the carbonate hardness of our aquarium. All these solutions go towards increasing the carbonate content of the water of the aquarium, but in this way they also indirectly stabilise the value of the pH.