Have you set up an aquarium and then noticed that the pH of the water continues to rise? Could rocks or gravel that were added as decor in the aquarium affect the water pH? Yes. If your rocks are actually limestone, they are the cause for the pH elevation in your aquarium water. Limestone is calcareous (contains calcium) and is known for its ability to both harden the water and increase the pH. In fact, if someone asks how to raise water pH, one method we suggest is to place crushed limestone, coral, oyster shell or any highly calcareous material in the filter.
Your local tap water may also be naturally hard (containing calcium and magnesium) and alkaline (containing carbonate), which adds further to the high pH. We'd definitely recommend testing the rock to see if it contains calcium carbonate and if there is any doubt, remove it entirely. If the tank pH stabilizes, you've found the culprit.
If you don't want to give up on the rocks you have, you should determine what they're composed of. Test the composition of your rock by putting a few drops of ordinary white vinegar on it. If the vinegar foams, the rock is calcareous and this will affect the hardness and pH of the water. Limestone is probably the most frequently encountered calcareous rock, followed closely by marble.
Another means of testing an unknown rock is to fill a bucket with water from the water source you plan to use. Then test and record the pH, hardness, nitrate and phosphate. Place the rock in the bucket and let it soak for a week, then test the water again. If there are no changes, the rocks are not likely to cause a problem in your aquarium. If the parameters change considerably, we'd advise against using the rock in question in your aquarium.
Although pH can be altered, we don't advise attempting to make major changes in the pH of your local water. It is very difficult to keep the pH stable over time, and you may be faced with an even more dangerous situation as a result: fluctuating pH. Changes in pH are stressful—if not lethal—to your fish. Furthermore, sudden changes in pH can damage the beneficial bacterial colonies that eliminate wastes in your aquarium.
In other words, keeping the pH at a steady state is just as important as the actual pH value itself. We suggest choosing fish that thrive in the pH of the water source that you have or find a water source that is already in the range you desire, rather than attempting to make major changes in the pH. If your water is naturally hard and alkaline, choose fish such as African cichlids that thrive in that environment. If your water is naturally soft and acidic, consider fish such as members of the popular Tetra family, almost all of which relish that type of water.
Over time, the pH in the aquarium water will change due to the chemicals produced by the fish and bacteria in the aquarium. It is important to perform regular partial water changes in your aquarium to remove the old water and add new water that will be at the normal pH again. For local water that is low in hardness and alkalinity, using some calcareous rocks in the aquarium will raise the hardness and alkalinity and stabilize the pH.
Zahangir M, Haque F, Mostakim G, Islam M. Secondary Stress Responses of Zebrafish to Different pH: Evaluation in a Seasonal Manner. Aquac Rep. 2015;2:91-96. doi:10.1016/j.aqrep.2015.08.008