Updated: Oct 3
#Rainwater will not have a pH of 7/9, normally, Clean rain has a pH value of between 5.0 and 5.5, which is slightly acidic. No matter which direction it comes from, and how many days it will come. Normal rainfall is slightly acidic because of the presence of dissolved carbonic acid. Carbonic acid is the same as that found in soda pop.
Also, It will always be Mild acidic because of Water: H20. Air contains nitrogen ( N ) and Oxygen (0). If the waterfalls at a high speed, then the air molecules disintegrate slightly. Some combination produces HNO3 i.e. nitric acid.
This acid makes rainwater acidic. But pure water is also mildly acidic due to its high content. Pollution is not necessary for this. By the way, sulphuric acid and nitric acid are also formed due to pollution.
Other Important Note: Don't add the bore water directly in tanks when fishes are in #Biofloc tank. If the percentage of carbon dioxide is more in the tanks then the pH will be reduced because in the presence of CO2 in water increasing the acidic.