Don't scrub the rock. The more you disturb it like that the more the tank will tend to get algaes. Use a turkey baster and blow the rocks off twice a day. Test your water for nitrates, and phosphates. You can never do too many water changes. Water changes is pretty much the default action for any issue.
What are you feeding the fish? Are you running GFO at all? What kind of skimmer do you have?