Angel is correct, and it is easier to raise hardness and pH than it is to lower it. I use dolomite gravel, either in the substrate or filter. In the substrate, the problem is that it is there permanently (have to tear the tank down to remove all or some of it), but in the filter you can easily change the amount.
First to explain your falling pH. In an established tank, the biological processes work to lower the pH by acidifying the water. The degree of hardness--basically calcium and magnesium--in the tap water act as a buffering through carbonates, and if the tap water is say moderately hard, and you do weekly partial water changes of 40% or so, the tank will remain fairly stable at the pH of the tap water. The softer the water, meaning the less calcium and magnesium, the less carbonates, so the less buffering action and the pH drops normally due to the biological actions.
You can add hardness via dolomite or similar calcareous substances. Crushed coral works, as do marble chips; or so I'm told, I've never used these. But in my tanks I have half a cup of dolomite in the filter and it maintains a GH of 2 [tap water is zero] and a pH of 6 - 6.2 [tap water is 7 but with no hardness the tank remains acidic, and would be 5 without the dolomite].
You could take some tap water to the fish store and ask them to test the hardness just to know; make sure they test KH and GH and give you the numbers. Or you might contact your water supply people and ask, they often know and it may be on their website. This helps to know because you may not need much calcareous material to raise it to mid 7's. Using dolomite or similar is much safer because it is constant [my dolomite has worked for years] and less likely to fluctuations.
As noted previously, livebearers must have moderately hard and basic/alkaline water. Calcareous rock is the best way; limestone or lava rock in the tank would do some, but very minimally; the broken up dolomite is much more effective.