June, limestone affects pH by adding a buffer to the water to combat the lowering of the pH. Old tanks tend to get acidic. The pH will also drop with driftwood, rotten food/plants, or uncleaned waste.
You can use limestone, as 6.4 is bordering on too low. The pH should slowly climb and balance out. I would do water changes as well, maybe 10% twice a week. This depends on the pH of your tap water. If the pH of your tap water is low, it will not bring your tank pH up.