First off you need to see what the Ph of your tap water is. Fill up a cup of water and either aerate it or stir it off and on over the next 24 hours. Then test to see what it's true ph is after it has gassed off.
If your tap waters ph is low then you will need to add a buffer, such as crushed coral, to raise and stablize your ph.
If your tap water has a much higher ph than your tank then the buffers in your tank water are depleted. In this case you need to do a 50% WC
, then check the Ph. Keep doing WC
's, along with cleaning the gravel daily or every couple days until you slowly get your ph back up. You need to be doing weekly 50% WC
's to keep buffers in your water.