The Hydrometer

Aquarists can use a small arsenal of tests to assure themselves that the seawater quality in their fish only aquariums or reef aquariums is high enough. One of these tests is carried out using a hydrometer.

The hydrometer measures the Specific Gravity (SG) of seawater. SG is the measurement method used overall in the hobby, though more scientific circles could well measure in parts per thousand (ppt).

The hydrometer is a very simple piece of equipment and very easily obtainable for a reasonable price. It comes in two main types, the ‘swing needle’ and ‘floating needle.’ Both types are suitable for a marine aquarist.

The ‘swing needle’ type is usually a plastic box about four inches square, which is very narrow in width and marked with graduations on one side. Inside the box is a needle that can swing in a vertical direction. When the unit is filled with seawater the needle moves and indicates the SG.

The potential problem with these type hydrometers is that small bubbles could be attached to the needle and not be noticed by the aquarist. This will cause the reading to be incorrect. All that is needed is for the aquarist to gently tap the needle with a pencil of similar a few times and any bubbles will be dislodged.

The ’floating needle’ type consists of a cylinder, made of plastic or glass, and a narrow needle which is marked along its length. The cylinder is filled with the seawater to be tested, and the needle is floated inside the cylinder. The needle will float at a certain position, and the aquarist can read off the SG.

The potential problem with this type is that the aquarist will read the needle float point at an angle, and thus an incorrect reading will be obtained. So obviously it is important that the aquarist has his/her eye level with the top of the seawater in the cylinder.

Hydrometers that are specially manufactured for aquarium use are preset for warm temperatures, usually 75 deg F. This is because temperature affects the reading. Aquarist may keep their seawater at a different temperature to this, so the manufacturers often put in a chart to permit an easy adjustment of the reading to be made. These adjustments are often shown in marine aquarium books. However, there is no need to be overly concerned – what the marine aquarist is aiming for is stability, and if the reading of seawater at, say, 77 deg F is the same each week then there is little to worry about.

Another important point about hydrometers is that they must be kept clean to avoid salt deposits that could cause an incorrect reading. This is easily done – with the ‘swing needle’ simply fill it with tap water after use and leave for an hour or so, then empty and leave it to air dry. The ‘floating needle’ type can be the same – fill the cylinder above the level the seawater reached with tap water and let the needle float (it will not be supported by tap water) for an hour or so, then empty it out and dry with a clean cloth.

If I had to pick one of the two types of hydrometer, though there is not much to choose between them, I would go for the ’swing needle’ type. This is because it is probably a little easier to use and, at least in the case of glass, stronger than the ’floating needle’ type.

A hydrometer is an essential piece of equipment and every marine aquarist should have one. Specific Gravity should be tested at least weekly, and should certainly be in the aquarist’s testing routine.


The Hydrometer
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