Literally speaking, pH is a measure of the relative amount of free hydrogen and hydroxyl ions in water, but for our purposes we can say it’s a description of relative acidity/alkalinity in a liquid. Measurements range from 0 (extremely acid) to 14 (extremely alkaline), while a neutral pH is 7.0. Different fish can tolerate a wide range of pH, depending on factors like the water temperature, hardness, and their own native environment. A species native to rivers or lakes high in tannins, for example will most likely tolerate higher levels of acidity than most. Mollys, Guppies, Platys & Swordtails are varieties of tropical freshwater fish that can tolerate pH levels up to 8.5. Rasboras, loaches and hatchetfish, on the other hand, prefer water on the acidic side, generally under 7.0
While fish do vary in their requirements and preferences, it’s generally true that extremes in alkalinity (greater than 9.5) or acidity (less than 4.5) are dangerous and can be deadly. Extreme alkalinity can lead to the breakdown of cell membranes, while high acidity can affect reproduction and expose fish to higher levels of dissolved metals in the water. Plants also have their own preferences for pH. For aquaponics enthusiasts, it’s necessary to match the needs of your plant crop with your fish crop. Ever wondered why aquaponics is so dominated by tilapia? In large part it’s because tilapia can thrive in a broad range of environments, including pH ranging from 6.0 to 9.0 and plants generally do best with pH below 6.5. However, a critical part of a successful aquaponics system is the nitrifying bacteria, and it works best above 7.5 and pretty much shuts down below 6.0. Obviously, there’s some tension here between the elements, and the optimal range is the narrowest of margins between 6.8 and 7.0. That’s impractical to try to maintain, so for most aquaponic systems, a range of 6.4 - 7.4 is a more realistic target.
