Ista - Swing Arm Hydrometer
The device is very simple in both its design and use. The hobbyist dips the entire unit into their aquarium, filling it to the top with saltwater. The unit, now full, is then removed from the tank and set on a level surface. The swing-arm within the hydrometer will rise to indicate the specific gravity (also known as relative density) of the water sample. The more dense the water, the higher the arm will rise.
The are have clear indication of where the level should be and if your tank is in the higher or lower end. Please note bubbles can easily get stuck to the swinging arm of the device and cause an inaccurate reading.