If your tank has nitrates greater than 5 ppm and you don't do the dilution, the reading will peak at 5 ppm. Let's say your nitrates are really at 7 ppm, the reader will still just read 5. Unless you do the dilution it will not read above 5 ppm. Let's say you do the dilution step first, and then run the test. Your reading then should be 0.7 ppm...then multiply by 10. Hope that makes sense. And unfortunately the dilution step is a hassle if you don't have some fresh saltwater made up. And make sure the fresh saltwater matches the same salinity as your tank as best as you can.