You shouldnt need to add sand in order to keep your Ph stable... Ph will fluctuate normaly with the light cycle, usually higher diring the day and lower at night. If you want to stabalize your Ph, I suggest dripping Kalkwasser along with your auto top off and a reactor, many people find that a Kalk and Ca reactor is the ideal settup.. Its very important to continually monitor Ph when dosing Kalk, I suggest a Ph monitor rather than a test kit. If youre interested in more specific information on Kalk, Id be glad to help where I can...