Coral - January/February 2010

production and shell manufacture, respectively, or of trace elements such as iron and copper, without which photosynthesis would be impossible. All the salts taken together total, on average, 34. 7 g/L of natural sea water. Put another way, the average salt content, or salinity, of sea water measures 34. 7 percent.

When referring to the salt content of a sample of aquarium water, the commonly used terms are “salt density” or specific gravity. This is defined as the mass of all the dissolved salts (in grams) per milliliter of water. A seawater salinity of 34. 7 percent corresponds to a density of around 1.0232 g/ml (at a temperature of 77°F [ 25°C]). This brings us right away to another very important point for us marine aquarists: the density of sea water is dependent on its temperature. So a density figure without a corresponding temperature is useless. A short example of this: if 33 grams of marine salt are dissolved in a liter of water, resulting in a salinity of 33 percent, then at a temperature of 68°F ( 20°C) this gives a density of 1.0211 g/ml, but at a temperature of 71. 5°F ( 22°C), the same solution will have a density of 1.0227 g/ml, and at 77°F ( 25°C) it will be 1.0220 g/ml. This example clearly shows the effect temperature has on density, and points up how important it is for the aquarist to measure the density using an instrument that is calibrated to the correct temperature.

read the
salinity
here
Scale for
reading
salinity
Weight
Diagrammatic representation
of a hydrometer; the reading
is taken from the scale at the
water line (right), not where
the water’s surface contacts
the hydrometer.

on the scale where this elevated water level contacts the glass of the hydrometer, but at the point on the scale that matches the overall water level.

It is essential that the hydrometer be calibrated for the water temperature. Because this is generally around 75–79°F ( 24–26°C) in the tropical marine aquarium, one should buy a hydrometer calibrated to this temperature range. The hydrometer must be clean; if it is coated in algae or salt crust, it will produce incorrect data.

MARINE AQUARIUM WATER DENSITY

A density of 1.022–1.024 g/ml at a temperature of 77°F ( 25°C) is recommended for most marine aquariums. This corresponds to a salinity of around 33–35. 7 percent ( 33–35. 7 g/L of salt).

Hydrometers: Nowadays there are many suitable devices for determining the density of sea water. The hydrometer remains the simplest, the cheapest, and the most reliable for the beginner in the marine aquarium hobby.

D. Brockmann

The hydrometer can be placed in an area of the aquarium where the current is weak, but it is much better to do this in an external flask to avoid break and get more accurate readings. As soon as it floats steadily, the density is read off from the scale precisely at the water line (see diagram). Because of surface tension the water will extend somewhat higher on the glass of the hydrometer; however, the reading should be taken not at the line

Swing-needle density meters: Another simple density-measuring device utilizes the swing-arm principle. To use this type of device, one puts a sample of sea water into a container containing a plastic arm, which then swings to the point on a scale representing the relevant density. Extreme cleanliness is important with this device as well. Traces of fresh water or salt encrustation will lead to incorrect measurements. It is advisable to rinse swing-needle density meters several times before use with the sea water to be