Get your hands on some hydrochloric acid and test the rock you find suspicious. Limestone will bubble if you drop HCl on it. That could potentially raise your tank PH. I kind of think it won't raise it much, since I have a big chunk of limestone in my HOB filter and the pH doesn't go much over 6.5. My water is very soft, and I was trying to give the pH some stability.
Some substrates such as eco-complete can raise the Ph of the tank in the short term. I don't know if eco-complete still has this problem, but the effect is short lived, so I wouldn't worry much about it.
In any case, let the pH do what it's going to do. If the pH of your tap water is still 7.4, then just do a big water change before adding fish. You can also try adding driftwood, or filtering through organic peat moss. Take it easy on the changes if you have fish in it now.
Truth be told, fish can withstand out of range pH values a lot better than they can withstand their owner trying to mess with the pH all the time.