Wet Thumb

by Andy Horton 

Using a Hydrometer

A Hydrometer is an essential item of equipment for measuring the specific gravity, and hence calculating the salinity of the water. There are 2 types available.

 If the bulbous hydrometer is used, it should be placed in the aquarium water when the filtration is turned off, and the reading taken at the water surface level.

The specific gravity of water is denser at lower temperatures, and the reading on the hydrometer scale (calibrated at 15° C ) at the meniscus (curved upped edge of the liquid where it meets the stem) will be correspondingly higher as shown below:

The ability of fish to tolerate changes in salinity varies with different species. Invertebrates, especially Anemones will not thrive in reduced salinities. As a very rough guide a specific gravity reading of 0.001 corresponds approxmiately to a 0.1% alteration in salinity, and this variation is the tolerance limit. Water with a salinity of. less than 3.0% is defined as brackish; and in excess of 3.7% as metahaline, and likely to be found only in enclosed lagoons.

    Starfish and urchins are intolerant of salinity alterations

Synthetic marine salts should not be mixed in an inhabited aquarium. A fermentation bin, used in beer making is an ideal container for mixing the salts prior to changing a 20% maximum proportion of the water.

Seawater for the Aquarium
Wet Thumb (Marine Aquaria page)

British Marine Life Study Society Home Page
News 2019
News 2018
Main Links
Membership Form
Top of the Page