I use to have 3 or 4 those cheap clear acrylic, swing-arm style hydrometer.
And you know what no 2 of them would read that same.
I am sure that you know taking care of fish and corals requires very strict water parameters. The cheap hydrometer were and are a waste of time and money. Get yourself a Hand Held Refractometer w/ATC.
It is will worth the cost compared to the cost of your coral and fish.