Marine aquarists use a hydrometer to measure the specific gravity (SG) of the seawater. The hobby hydrometer is not a pure scientific instrument but is capable of sufficient accuracy to enable stability of the SG – provided the aquarist does at least a weekly check.
Marine aquariums come in two main types, fish only and reef. To be precise, for ‘reef’ I should really say mixed reef and corals only reef, the former being corals and fish (the usual).
The first consideration is what SG is really necessary. To answer that it would seem best to look at the wild reefs, our livestock’s natural home. The SG of the sea in those locations should give the answer. Well, yes it should, but it doesn’t seem as simple as that. Is it ever!
The sea is considered to be stable which for the volume taken up is not really surprising. The SG on the reefs though is perhaps surprising, as it can range from 1.017 to 1.031*. Specific gravity is affected by temperature and some variance in temperature is understandable. In addition, some seas are more ‘land-locked’ than others such as the Red Sea. So there is going to be variance.
Some state that it is best to mimic nature and I sympathize with that. However, mimic which reading?
Looking at averages could be the answer. The lowest encountered salinities (on reefs measured) had an average of 1.025, and the highest 1.026*. Now this is more like it.
Provided livestock are acclimatized properly there is tolerance to some change which over a period becomes the normal. Generally, fish are like this and will happily live in seawater with an SG from 1.022 to 1.026. (Some aquarists use lower SG levels for specific purposes.) There seems little point in having a higher SG than necessary so many fish only systems run at SG 1.022. There is some evidence that certain parasites don’t do so well at 1.022 so that is a bonus. Also, and very much secondary, not so much dry salt will be required for routine seawater changes which will reduce costs a little.
What of the reef system? Whether this is corals only or mixed, the corals dictate the SG requirement. Corals are much less tolerant of a lower SG and 1.024 is the minimum for them. The range usually cited for corals is 1.024 to 1.026.
Whatever the seawater SG is, it should be stable. Testing at least once weekly is required and unlike many other types there will not be a need to renew a test kit from time to time. The hydrometer, barring accidents, is a once only purchase.
SG is a very important parameter of seawater and one that is very easily controlled. The SG could reduce because of salt ‘creep’, where salt encrusts overhead wiring, lights, glass and the like. The SG could increase because of poor seawater top-up management, as only fresh water evaporates leaving the salt behind. High quality seawater is the often quoted number one necessity and simple monitoring is required.
(*Reference: Aquarium Corals. Eric H. Borneham)