I did a water test and everything is good except the nitrates are very high. I have done water changes and it's good for a day but then goes back up. How can I lower? Tank is a 125 gallon. Running fluval fx4 and an aquaclear 110. Have 28 Malawi cichlids (haps and peacocks) from 1-4 inches. I do water changes 40% every 2 weeks. Just ordered an eheim classic 600 for additional filtration. Any help is appreciated. Thank you.