We've been cycling a 53 gallon saltwater aquarium for about 20 days now, using a Red Sea Reef Mature Starter Kit for adding the nitrifying and denitrifiying bacteria. Our setup includes live rock, live sand, a protein skimmer, and a biofilter media block. We have been cycling according to instruction, and doing weekly water changes of about 4 gallons. A couple days ago we added our first creatures to the tank: 9 herbivorous snails. Recently, our Nitrates been gradually rising and are now at 30ppm. Our Nitrites and Ammonia are next to 0, and every other parameter in the tank is rather stable, including pH and salinity. For the water testing, we have been using API Nitrate test kits. What can we do to lower the Nitrate levels before it gets dangerous? And what could be the source of the problem?