Limestone will make your tank alkaline. So, if that's what your fish like...go for it.
You can't put just "any" rocks in your tank. The colloquial rock may contain different forms of copper, lead, mercury, iron, arsenic, molybdenum, sulphur, asbestos, etc. If you're unsure, take the rock to a state geologist or the museum and find out what's in a rock before putting it in a tank. Even cleaning some rocks can cause the release of detrimental materials into your household.
For the most part, you're OK with granites, agates, and quartz. But, as with everything else, do the research before adding anything to your tanks.