Just to elaborate on that if shes just drinking water,
ppms will go up instead of down , Ph may also stick or start to go down because the ec of the nutes will always be more acidic than the water.If shes taking all the water and leaving the nutes ph drops.
If ppms are dropping slowly thats ok shes fine no rush top up with some water at top up time.
If ppms are dropping fast but not to fast ph is rising slowly -this is exactly what you want.
if ppms are going majorly fast and ph is swinging up hardcore ,you have no buffers she wants more nutes. top up with some nutes.