Aquarium water will progressively become more and more acidic because of the following:
First, the fish and aerobic bacteria are discharging CO2, which is then converted to carbonic acid. Second, the fish and bacteria are discharging a variety of organic acids into the aquarium water. Obviously, the more fish, the greater the rate of acidification. Of course, you will want to continue with weekly partial water changes and this will help with the acification problem somewhat. However, the easiest, cheapest and most effective method for counteracting this acidification is to follow a weekly regimen in which sodium bicarbonate (baking soda) is added to your tank. For example, starting this Saturday, take an inital pH reading and then add a teaspoon of baking soda to your aquarium. Then take a second pH reading. Follow this regimen for one month and see how the pH reading fluctuates. Make adjustments to the amount of baking soda added so that you achieve a more or less stable pH reading within the range you desire. BTW....you should probably be changing 40-50% of your water each week in a heavily stocked tank.
