There are basically two kinds of hydrometers on the market: floater and swing-needle. They are very helpful, useful and inexpensive.

Specific gravity should be in the range of 1.021 to 1.024. It should be maintained within this range.


MECO(TM) Aquarium Glass Hydrometer Fish Tank Water Temperature Thermometer Salt Marine

This is the most common in the earlier days of saltwater aquarium. It looks like a large floating thermometer.

It has a scale inside a long-necked tube. The level at which the tube floats depends on how much salt is in the water.

The scale is read where the water’s surface meets the equipment.

The only difficult part is the water movement, making it hard to read the results accurately.


pranovo Sea Hydrometer Salinity Meter Specific Gravity Test for Aquarium Fish Tank Water Marine Sea Saltwater

This kind is actually easier to read than the other one. This is a small plastic box with a needle in it that pivots when the box is filled with aquarium water.

The needle will point to a scale, which will then tell you the specific gravity.

Swing-needles usually need to be calibrated to get accurate reading. So make sure that you follow the manufacturer’s specific instructions on how to use this equipment.

The needle can also sometimes stick. Tapping it will do the work.

This kind is also known as pointer or floating-needle.


Another way to measure salt water or the salinity of seawater is with a refractometer.

Salinity Refractometer for Seawater and Marine Fishkeeping Aquarium 0-100 Ppt with Automatic Temperature Compensation
  • Measure salt water or salinity of water, ideal for aquariums and marine monitoring.
  • Measures on 2 scales, Specific Gravity (D 20/20) and parts per thousand.
  • Features automatic temperature compensation.
  • Comes with hard case, dropper, screwdriver, user manual and cleaning cloth.
  • The main part is made of chromium, it's very durable.

The measurement is very easy. Place a few drops of the liquid on the optical part of the refractometer, cover the prism (the liquid will spread more easily over the prism), look through the eyepiece against the light (focus, if necessary) and you will be able to read the result value on the scale.

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