As we have mentioned in many articles on this site, pH is very important for your tank’s health. PH levels can vary over the course of a day due to the nitrogen cycle and natural biological filtration cycles at work on your aquarium, so small fluctuations are to be expected.
It is only when you notice a trend of pH imbalances that you should act. Most freshwater fish and plants tend to be happiest in the 5.5 – 7 range, though there is a lot of variance as this table shows between even freshwater fish:
As you can see, it would be difficult to keep rasboras and blind cave tetra fish in the tank because their preferred pH levels are quite different (5.8 to 7.0 and 7.5 to 9 respectively). This is not to say that it can’t be done at all, but there would have to be a compromise; it’s more than likely your fish could tolerate and even do well in a tank with a mean pH of 7.3 or something outside of what the book calls the ideal range. As long as the pH is kept consistent they will be able to adapt and thrive.
You should always check the pH of your tap water before making any adjustments to your water’s pH. Hard water is usually more alkaline, so you may need to add some peat moss or almond leaves to pre-treat your water before adding it to the tank. If the water is soft, consider adding an acidic component like driftwood. You could also buy a reverse osmosis system which should help keep the pH of your water around 7.0.