Generally, it is safe to say that you can leave the pH levels alone in your aquarium. However, if you are finding that the pH levels are changing and that they are altered a good bit from tap to aquarium, you might want to find out why this is. There could be a big problem that you are going to want to address. You need to know that there are generally going to be three causes of a pH shift. There could be something that is in the water source that is causing a buffering of the water. There could be something within the fish tank itself that is causing the buffering of the water. There could also be something that you are directly adding to the fish tank that could be causing the buffering.
Even though it is very unlikely that it is the water source, such as the water pipes in your house, that are causing the shift in pH levels, it is one of the easier things to test for so you might want to cover this first. All you have to do is to fill a bucket with the tap water and immediately test the pH level. Allow the bucket of water to sit for many hours and then test it again. If you notice a change in the pH level, then you can assume that it is the water source that is the culprit. However, if there are no changes, then it is time to move to the next test.
Now, there could be something that you have in the fish tank that is causing the pH levels to change. Things that are commonly known for suddenly changing pH levels would be that of seashells, wood, dead plants, peat, coral skeletons, dead fish, limestones, bones and leaves. If you have any of these in your fish tank, you are going to want to remove them right away. If you are unable to detect anything that would cause higher pH levels, then you might have to look a little closer, particularly at your substrate. Crushed coral is usually meant for marine aquariums where the higher pH is generally desired. If that does not seem to be the problem, you are going to want to remove everything from the fish tank that does not need to be there. Allow the fish tank to sit for several hours and then test the pH level again. If it goes back to normal, then you know it was something you had placed in there.
Most commonly, the reason behind the change in pH levels is the addition of chemicals. These are generally chemicals that did not need to be added in the first place. Sometimes, chemicals will react badly together and they will cause an abrupt change in the pH level. In order to determine whether it was any of the chemicals that you are using, get a bucket of water and treat is according to the amount of water that is in the bucket. Allow the chemically treated water to sit for about 15 minutes and then test it. If it tests for an altered pH level, then you know what the problem was. Most likely, the chemicals you are using, for one reason or another and that are altering the pH levels, are chemicals that your fish can live without, so stop using them.