I believe hydrometers can be accurate, I was worried because measuring salt by volume can throw off salinity/specific gravity because water can get absorbed into the salt and change the volume/mass/density.yeah salinity is perfect and when i checked the tank it was in before the 100% water change it was perfect too. i measure in specific gravity with my hydrometer more than actual salinity because it has a more exact number where as salinity is always kinda between things. my specific gravity before and after was 1.025 which is perfect for coral and macro algae.