There are parameters that need to be monitored in any marine aquarium, though reef aquariums need the most tests. One check applicable to all marine systems is salinity which should be checked weekly.
The instrument used is the hydrometer, which sounds very scientific but in fact is simple and easy to use. It must be one of the quickest tests that the aquarist performs.
The hydrometer can only perform properly if it is in good condition. Any instrument that is damaged is suspect. Similarly, if there is a large change in the hydrometer reading to the previous week perhaps there is something amiss with the instrument. The best way of checking a hydrometer is to use it against another – an aquarist colleague can use his/her hydrometer and then the suspect one and compare, or the same thing can be done at the local retailer.
Usually there isn’t anything wrong with the hydrometer but the reading is being affected. With the ‘swing needle’ type instrument it can be salt deposits on the needle or the pin area. This can be avoided by leaving the hydrometer after use for an hour or so full of tap water, then emptying it out and allowing the instrument to air dry. If the hydrometer is the tube and floating needle type, again salt deposits can accumulate around the floating needle, sometimes where the water surface reaches. The solution is the same – leave the tube full of tap water and put the floating needle in. Be careful as the floating needle will sink. Leave for an hour or so and empty, wiping the floating needle dry with a clean cloth.
If the hydrometer seems to be giving an unusual reading, before finishing the test try the following. This should be done routinely at each test anyway. When using a ‘swing needle’ type make sure there aren’t any air bubbles attached to the swing needle by tapping it gently a few times with a pencil or similar. With the tube and floating needle type, make sure the reading is being taken with the eye horizontal to the top of the seawater surface, otherwise the angle could cause an incorrect read-out.
Also remember that if the aquarist has adjusted the aquarium temperature for any reason, this can affect the hydrometer reading.
It is unlikely that the hydrometer will fail as they are such simple pieces of equipment, particularly the ones designed for the hobby. All that is usually required is for a simple precaution to be taken when in use, such as the described lack of bubbles or a correct viewing angle, and also that there aren’t any salt deposits before the unit is stored away.