the only difference is saltwater fish are a little mor sensitive then freshwater. from what i've read freashwater fish can live with nitrates of 80ppm while saltwater can only cope with abotu 40ppm
I've read articles that state to keep it under 80ppm, and even some that said under 100ppm. I know that I've had about 100ppm a while back after a large amount of cyano dieoff. The fish seemed fine during that, even though I got it down within a week. 40 is bad, but I don't think it's a dangerous level.