The Coralife Deep Six Hydrometer is a great tool for taking aquarium water samples at six inches deep. Without having to insert a hand, you are greatly limiting the chance for potentially contaminating the water. It has been proven that taking water samples of six inches deep is more effective at giving an accurate salinity reading than with surface samples. Each sample unit has been calibrated, tested and certified for accuracy and reliability. For most brackish fish, a specific gravity of 1.005-1.010 is adequate enough.