Hydrometers and Refractometers

There are 3 basic ways to measure the salt concentration in your aquarium. You can call it salinity or specific gravity and there different scales. Here I would like to explain the pros and cons of glass hydrometers, plastic hydrometers and refractometers.

Glass Hydrometers
There is the inexpensive, shorter, 6″glass model with a section of the stem marked in green. They are difficult to read and impossible to get an accurate value. Forget this one.

The absolute best hydrometer for accuracy is the 160mm certified laboratory hydrometer graduated in 0.001 increments. This is what we use and sell. This type of hydrometer is used to calibrate all other hydrometer types and refractometers. They are somewhat awkward to use and break easily. We include a 250ml plastic cylinder you can fill with saltwater to make reading easy.

Plastic Dip and Read Hydrometers
When calibrated this is your best choice. Fast, easy and cheap. Uncalibrated it can be a disaster. Before we sell plastic hydrometers, they are soaked in saltwater for 2 weeks, calibrated with a laboratory hydrometer and the correction amount is written on the front. If it is stored with water in it, fresh or salt, it will stay calibrated. Directly out of the package, micro bubbles will stick to the float and give a false high reading.

The Deep 6 model advertises +/-0.001 accuracy but can be +/-0.003. Instant Ocean is the worst averaging 0.005 low.

If you would like your plastic hydrometer calibrated, old or new, mail it to us with a prepaid return mailer.  We will write in permanent marker the amount to add or subtract from the actual reading and the date. Hydrometers that are greater than 0.002 off will be calibrated but should be replaced. Please soak new ones in saltwater for 2 weeks. Re-calibrated yours every 6 months.

They are not as fast as a plastic hydro meter and care must be to rinse with TDS=0 water and dry between measurements. Testing many aquariums can be slow. You must be absolutely certain your “zeroing” water is TDS=0. TDS meters are sensitive but they require calibration too. If you have to calibrate an instrument to calibrate another instrument the possibility of error is huge. You should purchase laboratory standardized saltwater to check your Refractometer. Dropping Refractometers can ruin them permanently and they are not cheap. It’s possible to zero an inaccurate Refractometer.

When reading a hydrometer the first rule is to read horizontally across the top reading at the top the meniscus.  Be sure the hydrometer isn’t touching anything.


It is common to read at the water level for liquids less dense than water (<1.000} and at the top of the meniscus for liquids more dense than water (>1.000).  Saltwater should be 1.023 to 1.024.

Most website’s recommend reading at the water level which is incorrect.

The meniscus happens because of water‘s surface tension


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