The salinity of the marine aquarium is one of the major parameters that must be routinely checked. Salinity can change because of evaporation, and it is important to keep it stable. Daily top-ups of the water to a pre-determined level is an easy way, or using an auto top-up device.
Aquarists normally deal with [tag-tec]specific gravity[/tag-tec] (SG) which is a straightforward measurement to obtain. The instrument used is a [tag-self]hydrometer[/tag-self], normally of the swing needle or floating needle type. They are both accurate enough for hobby purposes, and any small inaccuracy can usually be disregarded as long as the measurement is stable.
On the wild reef the [tag-self]salinity[/tag-self] measures an average 35 parts per thousand (ppt) *. 35 ppt is the equivalent of close to SG 1.026. It would seem therefore that an SG of 1.026 should be maintained. There are many aquarists who consider this to be correct, stating that mother nature knows best. These aquarists include several well known names, and they are concerned with captive reef systems.
The fish only aquarium can be kept at a lower SG. In the many years that aquarists have kept fish only systems, and remember these existed well before captive reefs, it was found that there were advantages in maintaining a lower SG. First of all, there was evidence that certain fish parasites do not do well at a lower SG. This is supported to an extent by one method of dealing with parasite infected fish – lowering the SG to destroy them. However, the SG being considered here is not as low as that. The level is 1.022. This will mean that the fish hopefully will not fall foul of parasites so easily, and the cost of salt to the aquarist will be lower (though the cost is not the most important consideration). It has also been stated that the marine fish natural bodily function of water transference is less strenuous. There is no evidence that maintaining fish at a lower SG is detrimental to them.
SG in a captive reef system is not so straightforward. A higher SG should be maintained as there is evidence that some corals may not open polyps in a lower one. Corals are also stressed over the long term by a too high or worse too low SG and could even die if the situation was allowed to continue. As said this is long term – changes in SG of a short duration, though very undesirable, will not have the same result. It is reported that soft corals are more influenced by incorrect SG *.
I have a soft coral reef that I do not maintain at 1.026. It is maintained at 1.024. This level does not fluctuate by any amount to speak of. The corals are healthy and growing and there are, and have never been, any signs of stress.
So, an aquarist keeping a fish only aquarium can safely reduce the SG to 1.022. There is no reason it could not be higher, after all the fish live in the wild at a higher level, but it is well known that a lower SG will do no harm.
Where there are corals involved, it will be best to maintain a higher level of 1.026. The aquarist with the confidence of experience can lower this SG very carefully over a lengthy period, and note if there are any bad reactions by the corals. If there are, the SG must be raised, again over a period. Sudden changes are not wanted. If there are no bad reactions, then SG 1.024 is probably the lowest that should be maintained.
If aquarists are able to take their swing needle or float needle hydrometers to somewhere that can test the accuracy against a known correct measure, all the better. The aquarist can then add to or take from the required measurement on their hydrometer to obtain more accuracy.
( * Reference: Eric Borneman. Aquarium Corals)