water changes are actually pretty ineffective at reducing nitrates... a 10% w/c would take you from 30 to 27. Larger water changes are helpful, but IMO it's best to attack the root of the problem. examine how much and what you're feeding and, generally, cut back. Make sure your skimmer is functioning correctly. Grow macro algae, or consider a deep sandbed somewhere in your system (a remote DSB is usually easier long term)