Look around for anything that died and is lodged in the rock somewhere.
Do a 10% change now. Start aerating/aging water for another 10% change in a few days.
Also, are you using the same test kit or are you using a digital pH meter? With some kits if you accidentally leave the cap off or loose, it won't work right and you will get false readings (happened to me). I suggest confirming with a brand new kit.....or just buy a digital meter.
I always do a 10% water change weekly. I feel it keeps the system more stable than 20% changes every 2 weeks.
Also, test the pH and alkalinity of your new water. Perhaps your salt mix is a little off.