The ideal range for pH in an aquaponic system is between 6.5 and 7.0. But why? In an aquaponic system, there are three distinctly different living inhabitants: the fish, the plants, and the beneficial bacteria. Therefore, it is important, we pay attention to the specific needs of each living part. When thinking about them individually, each has a slightly different ideal pH range. Most species of fish favor a slightly higher pH and most vegetables favor a slightly lower pH. The ideal pH for most of the beneficial bacteria in the system is in between these two.
A pH range of 6.5-7.0 is the compromise between the needs of the fish, beneficial bacteria and plants, creating an environment that is hospitable of each of the three essential living parts to an aquaponics system.
How does a pH outside of these ranges affect the health of the system? If system water becomes more basic and pH levels start to rise, it will limit the plants' ability to uptake specific nutrients needed for healthy growth. For instance, at a pH of 7.8, the availability of elements such as iron, boron, and manganese decreases, even if they are available in the water. On the opposite end, if pH were to decrease and become more acidic, you would notice a difference in the performance of nitrifying bacteria. At a pH of 6.2 nitrification will begin to slow down, leading to increased ammonia and decreased nitrate levels.
Now, think about why pH would change throughout the life of an aquaponic system. It is not uncommon to see the pH of system water decrease over time as the system becomes more mature. There is a scientific reason why this is happening. During the nitrification process, certain byproducts are produced, one of which is nitric acid. As nitrification occurs throughout the system, the naturally occurring nitric acid will begin to lower the pH from its original starting point. It can take a year or more in larger volume systems to see a pH stabilize after these changes. Because pH is such an important water quality factor, is vital to test very frequently.
Source: Aquaponics.com