No3 levels in the ocean are at constant detectable level that never gets depleted by products like NoPox. You’re comparing an open environment with unlimited nutrients to a closed one with a limited nutrients. This is why tanks that run levels exactly at NSW fail unless continually replaced on a daily, if not on an hourly basis with water that has trace nitrates and phosphates. Nutrient levels in NSW get depleted very quickly due to being so low in the first place. I live in San Diego and we have “Scripps water” out here. It’s NSW sources from the end of Scripps Pier and supplies Scripps aquarium. It’s also free to the public via a spigot and hose that has a knack of being stolen on a regular basis, lol. Reefers out here even go as far as doing constant water changes using Scripps water and some have large holding tanks full of it at their houses that they replenish periodically using a truck with a large tank in the bed.