Shop Floating Hydrometer

As modern hobbyists, we sure have it easy compared to days past. Digital testing equipment that produces accurate results and reduces the risk of human error during the testing procedure saves us both precious time and money. Technology like this certainly has its place in the hobby but sometimes, there is just is no reason to change what is not broken. The Tropic Marin Floating Hydrometer is most certainly one of those scenarios.

Refractometers are hands down the most widely used method of measuring specific gravity. They work great, are fairly simple to use, and can generally provide both specific gravity and salinity measurements. The downside is they can fall out of calibration easily, requiring frequent checks and re-calibration. Furthermore, traditional analog refractometers also need a substantial natural light source to take a measurement.

Floating Hydrometer

Floating glass thermometers have been around for decades and will provide users with quick and accurate specific gravity readings almost instantaneously. Tropic Marin's version is coveted for being highly reliable because every single one is made of hand-blown glass and calibrated before packaging. It never requires additional calibration checks and will continue providing accurate measurements for many years when cared for properly.

Get Some valuable tips for using Tropic Marin Salt Mix and learn how to choose the right salt for your reef tank.

Being made of glass, the unit is fragile so breakage is a small risk and it does not have ATC - automatic temperature compensation so the temperature of the saltwater your measuring matters. This hydrometer is calibrated at 77°F meaning the water your measuring needs to be heated to 77°F (or as close as possible) for an accurate measurement. You can get away with a usable measurement as long as the sample is within 4-5 degrees of 77°F.  If your water is really cold (or hot), you need to adjust the water temperature accordingly before taking a measurement but you can also just calculate using the appropriate conversion factor or better yet, an online calculator


  • Maximum deviation: 0.001 at 77° F (25° C)
  • Scale from 1.021 to 1.031
  • Resolution: 0.0001

Salinity measurement using a hydrometer

How To Use The Tropic Marin Floating Hydrometer

  1. Fill the measuring container with saltwater (water temperature = 25 ° C / 77 °F). A 100 mL graduated cylinder is perfect.
  2. The hydrometer should be dry and clean.
  3. Gently place the hydrometer into the water until it floats freely. A quick spin will remove any bubbles. 
  4. Now read the specific gravity on the blue part of the scale of the hydrometer at the level of the water's surface - DO NOT reference the edges of the meniscus creeping higher above the waterline.
  5. To avoid any distortion of future measurements, rinse the measuring cylinder well with fresh tap or reverse osmosis water to remove salt residues after each use.

**Pro tip: Do not measure directly in your display tank. Water movement of any kind makes it difficult to read correctly.