Question: Do rocks affect the water pH?
"I just purchased a 55-gallon tank. It has been up and running for three weeks. I found some flat rocks along a highway. Boiled them before putting them in the tank. I think that they might be limestone. I have just gotten the ammonia to (0), but the nitrites are high, changed some of the water out and added salt at a rate of 1 teaspoon per 10 gallons of water. However the Ph is very high at least according to my test kit chart at 7.6 or higher. Are the rocks causing the Ph to stay so high?"
Answer: Yes, if your rock is actually limestone, odd are good that it is the cause for the pH elevation in your aquarium water. Limestone is calcareous, and is known for its ability to both harden the water and increase the pH. In fact, if someone asks me how to raise water pH, one method I suggest is to place limestone, crushed coral, or any highly calcareous material in their filter. Your water may also be naturally hard and alkaline, which adds further to the high pH. I'd definitely recommend testing the rock, and if there is any doubt, remove it entirely. If the tank stabilizes, you have found the culprit.
If you don't want to give up on the rocks you have, you should determine what they are composed of. To test the composition of your rock, put a few drops of ordinary white vinegar on it. If it foams, it's calcareous, and 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 the effect an unknown rock will have on your aquarium is to fill a bucket with water from the water source you plan to use. Then test and record the pH, hardness, nitrates, and phosphates. Place the rock in the bucket, let it soak for a week and 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, I'd advise against using the rock in question.
You haven't mentioned what kind of fish you are planning to keep. Although pH can be altered, I do not advise attempting to make major changes in the pH of your water. Over time it is very difficult to keep the pH stable, and as a result you may be faced with an even more dangerous situation - 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. For that reason I 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 members of the popular Tetra family, almost all of which relish that type of water.
Lastly, the nitrite level is expected, as you are three weeks into a newly set up aquarium. The rocks are not causing any increase there. Occasionally some aquariums get stuck on the nitrite portion of the nitrogen cycle, but odds are you will see nitrites drop within the next week or two. Make sure you keep an eye on the nitrite levels, as they can be lethal if they get too high. Perform regular water changes to keep them from raising to dangerous levels. Once the nitrites fall, the aquarium should be relatively stable, and will remain so as long as proper maintenance takes place.
Good luck with your new tank. Send a photo when you get stocked.