The Nitrosomonas bacterium feed on ammonia and converts it into nitrite (NO2). Nitrite is slightly less harmful to fish than ammonia, but it is still not good to have around. Nitrites make it more difficult for fish to take up oxygen (O2). Thankfully, there is another bacterium that helps with this stage of the cycle. The Nitrobacter bacterium feeds on the nitrites in the water and converts them into nitrate (NO3). Fish can tolerate high levels of nitrate much better than high levels of ammonia and nitrite. Plants are very helpful in the nitrate phase of the nitrogen cycle because they feed on nitrates. Once a system has a sufficient number of the aforementioned bacteria to fully process the ammonia and nitrites, the system is considered to be “cycled.” This brings us back to pH. If the pH drops below 6 in a given system, the Nitrosomonas bacterium will not adequately process the ammonia in the water. If the pH eventually gets low enough, the nitrogen cycle will cease to function all together. If the pH levels rise above 9, as mentioned earlier, very high concentrations of toxic ammonia (NH3) are present. The nitrogen cycle itself tends to reduce the pH of a system. Most freshwater fish would fall into a pH range between 5.5 and 7.2, with a few exceptions at both the high and the low end. It is not just fish that need to have an appropriate pH to thrive. Live plants do best in a pH range between 6.5 and 7.5. Beneficial bacteria prefer a slightly higher range, from 7.0 to 8.0. In general, a good pH range that would satisfy most fish, along with live plants and bacteria would be 7.0 to 7.2. It is important to note that pH in the aquarium is not completely stable, with small fluctuations occurring throughout the day and night. Many things can affect the stability of the pH within a system. Aeration, gravel, fish tank decorations, temperature, and nitrate levels are just some of these.,What Is pH?