Any Idea What Could Be Causing The Ph In My Tank To Lower?

[The Taffy Apple]

I did a 40% water change and the pH of the tank came up to 6.8.
About 30 hours later I tested the tank and it is back down to ~6.0...

Should I try using some crushed corral? If so, how much should I add at a time? It is a 100gallon tank.

Did you test your KH (thanks TwoTankAmin )...?

If that result comes back similiar to mine then maybe it could be worthwhile trying crushed coral.

Terry.