Planted Tank Enthusiast
Join Date: Sep 2009
Location: Seattle, Washington
Do you have rocks or something in your tank? They could be dissolving and raising your pH. Unless you're adding things to your tank (aside from food, which shouldn't be causing that problem) your tank is a closed system, so there's basically no way the pH could be changing that much unless something in the tank or filter or somewhere in the system is causing it.
Also, do you know how old your pH test kit is? It could be a faulty test kit, although with your plants melting that doesn't sound like it's the case (unless they died for a different reason).