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Not true, you are correct that for the most part it will be non toxic to marine fish at the levels we would see in our systems.. BUT significant, severe elevations can still throw off homeostasis even in marine fish.
Check out Noga, 2010. He gives one example where the LC50 of spotted sea trout in 16 ppt seawater was 980 mg/l NO2-N. The chloride ion, in greater than a 2:1 ratio, blocks the toxicity of nitrite. 1 ppt seawater (barely brackish) contains 500 mg/l chloride. so would render nitrite non-toxic on the order of 250 ppm. The highest level I've ever recall measuring in an aquarium is around 1 ppm.