pH

Most people are familiar with terms like "acids," "pH," and "alkaline water," but what do they really mean? To grasp the concept of pH, it's helpful to start with a closer look at water itself.

Though water might seem simple and stable, it’s actually quite dynamic at a molecular level. Water molecules are polar due to the electromagnetic pull of oxygen on the hydrogen atoms. This creates partial charges on both sides of the molecule—hydrogen becomes partially positive, and oxygen becomes partially negative. These charges are strong enough that water molecules can sometimes "break apart," with one water molecule tearing a hydrogen off another. This process happens constantly in water, meaning that individual molecules are continually breaking and reforming.

So, what does this have to do with pH? pH is essentially a measure of how many hydrogen ions (H⁺) are floating around in a sample of water. In pure water, these hydrogen ions exist in equal numbers to hydroxide ions (OH⁻), and the pH tends to be neutral—around 7. At this point, the water is relatively unreactive because the hydrogen ions are paired up with hydroxide ions.

However, real-world water isn’t just plain H₂O. Because water is polar, it can dissolve many substances, which can alter the balance of hydrogen ions and hydroxide ions. Some substances release hydrogen ions into the water, making it more acidic (lower pH), while others absorb hydrogen ions, making the water more alkaline (higher pH). So, water with a pH below 7 has an excess of hydrogen ions, and water with a pH above 7 has more hydroxide ions. The further from 7 the pH moves, the more reactive the water becomes.

Why Does pH Matter for Fish?

Now, why should you care about pH when it comes to your aquarium? Fish, unlike humans, can't choose the pH of their environment—they live in the water we provide. This makes the pH of your aquarium water crucial to their health. Fish gills are especially sensitive to the water's reactivity. If the pH is too low or high, it can interfere with their ability to breathe properly and may cause long-term damage.

Most freshwater fish species thrive in water with a pH between 6 and 8, saltwater likes a smaller range of 8.1-8.5 or so. However, there are exceptions. Cichlids, for example, often prefer slightly higher pH levels. For instance, Lake Tanganyika, home to many cichlids, has a natural pH of around 9.

While the specific pH value is important, the stability of that pH is even more crucial. Fish are better off in a tank with a stable pH of 8.4 than in a tank where the pH fluctuates between 6.5 and 7.5. Sudden shifts in pH can cause stress, even if the overall pH level isn't extreme.

Several factors influence pH, some of which are easy to measure and others that are less straightforward. Understanding how to maintain a stable, healthy pH will help ensure your fish stay healthy and stress-free.