One of the most common problems encountered by the aquarist is maintaining a constant pH level. Many hobbyists fail to understand the importance pH plays in the aquarium and what factors influence the pH reading. The Discus is a very sensitive fish. You should be very careful about fluctuating ph in your discus tank. I have a personal experience I lost my few discus due to fluctuated ph level in my early days in this hobby. You can fix it at 7. You may use a ph meter to measure it.
When considering pH, you should have a goal of stability. Although 7.0 may be the standard pH level for a discus fish, the same fish will likely thrive at a constant level anywhere between 6.6 and 7.4. It may even survive at constant levels between 6.2 and 7.8. Our goal as aquarists should be to establish a very consistent pH level, even if that level is slightly outside the “ideal” pH reading for a given species. In other words, a constant pH of 6.6 is better than a pH value which fluctuates between 6.6 and 7.0, even for a fish which prefers a 7.0 reading.
This has been a brief overview of water chemistry as it relates to discus care. This only covers the high-level basics. There are many more points where I don’t give spotlight here. These conditions can be changed when you have a breeding pair of discus.

Hence by these points you know that how to obtain a fixed water chemistry level in your aquarium. Now the main challenging matter comes -> how to maintain it. When your aquarium is comparatively new then you have no chemical or biological wastes in your tank but when you add discus or any other fish and feeding them then chemical and biological processes comes into play and degrade water quality over time. These processes include the production of acid by the nitrifying bacteria in the gravel and filter, which means that over time the pH of aquarium water, will always tend to drop. Others are the constant formation of nitrate by these same bacteria and inputs of dissolved organic compounds from fish waste and uneaten food. The way to combat these changes and keep the water chemistry stable is to have an efficient, well-maintained filter system.