Hydrometers are critical to the health of every reef aquarium.  We need to keep the salt level within a healthy range (1.024 for reef tanks) and we need to keep it consistent - no swings up or down.  Cheap plastic hydrometers tend to give false readings over time as the "swing arm" can get weighted down with minerals.  This would allow one to believe their salinity is fine when in fact, it's much higher!  Don't risk the health of your entire livestock population!  Invest in a good quality, simple to use refractometer & rest assured you're getting a true reading!  I personally use (& love!) the below Premium Blue model & highly recommend it's purchase if you don't already own one!

The RHS series are designed for testing the concentration of saltwater and brine. It's triple scale provides a direct reading of the specific gravity and concentration. (Parts per thousand) of salt in water.   Easy to use Dual scale includes specific gravity and % per thousand Automatic temperature compensation Easily calibrated* Adjustable eyepiece No batteries required Includes pipette and calibration screw driver Range measurement:0-10% Minimum division: 0.10% Accuracy: 0.1%