What Salinity Should I Keep My Marine Aquarium At?

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)

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Water Salinity

Everyone who goes to the seaside for a day out or a holiday and has a paddle or a swim knows that a mouthful of water is salty. This saltiness is important, and the level of it.

Livestock that comes from the wild reefs is used to stability, and this stability requirement also applies in the aquarium.

The measurement of salinity in professional circles is usually in parts per thousand (ppt). In the marine aquarium hobby the measurement commonly used is specific gravity (SG). This is easily and quickly measured by the use of a hydrometer. Hydrometers are specifically made for the hobby and are easily obtained.

The SG scale that is of interest to the marine aquarist is from 1.022 to 1.025. For special reasons aquarists have run at a higher or lower SG than the scale given, but this will not be gone into here.

The initial consideration is the aquarium system itself: is it a fish only aquarium or a reef aquarium? If it is a fish only aquarium then running the SG at 1.022 is reported to be advantageous. This is because the fish are not physically ‘stressed’ as much as they would be at a higher SG – the outer surface of the fish body permits the passage of water (osmosis) and the fish automatically has to work to compensate for this. The lower SG reduces this process somewhat. In addition, it is reported that certain fish skin parasites do not do so well at the lower SG, and anything that makes life more difficult for a parasite is welcome. The lower SG also means that there is less dry salt required for routine seawater changes, meaning costs are lower, though this is not a priority consideration.

For the reef aquarium (no fish) and the mixed aquarium, it is probably better to run at an SG of 1.024 (or 1.025). This is primarily for the corals, as reports have indicated that they do not do so well at a lower SG. This applies to both soft and hard corals. My soft coral reef has been running at 1.024 for years.

If the corals are known to have come from the Red Sea, where the salinity is higher, it will probably be better to have the SG at 1.025 at least to begin with, or even 1.026.

There isn’t a reason why the higher Red Sea SG cannot be lowered carefully, but this must be done slowly. A sudden reduction in SG is not good. Reducing the SG from 1.025 to 1.024 could be done over a fortnight or more. Corals have evolved over a very long time and must be watched for any adverse reaction. An SG lower than 1.024 is not recommended for a reef aquarium.

Of course, when discussing the suitability of corals and SG, two things come to mind. The first is that perhaps the coral has been home propagated. If so, it will already be acclimated to a normal aquarium environment. The seawater SG is easily discovered by a simple enquiry. Second, a coral imported from the wild reef will usually be seen in a dealer’s display tank. Is the coral open and extended, looking beautiful? If so, what is the dealer’s SG? How long has it been in the tank?

It is not likely that the seawater in the dealer’s display tank and that in the aquarist’s aquarium will be matched, there could be differences in pH, temperature, SG, alkalinity etc. Therefore it is of great importance that the introduction of livestock – of any type – to the aquarist’s seawater is done properly and over an extended amount of time. This will allow at least some adjustment to be made. Livestock is very resilient given the chance, despite the fact it comes for the most part from a very stable environment, the wild reef.

Once livestock is in the aquarium and settled, it is important that the SG continues to be stable. First, routine seawater changes should be at the same SG and the same temperature. Second, any evaporation from the seawater needs to be topped up. Seawater should not be used for these top-ups, but fresh water, preferably reverse osmosis water. This is because the salt does not evaporate, and if seawater were to be used then the SG would slowly get higher as more salt is being added. Topping up can be automated for a large aquarium if desired, or done manually once a day for smaller ones.

There are some basic and important parameters that need to be monitored and kept stable, and salinity is one of them. A check with a hydrometer is recommended at least weekly.

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Quick Tip – Help Maintain Stability By Keeping Your Hydrometer Clean

When you start a saltwater aquarium one of the things which you must do is maintain stability.

The hydrometer is an essential item when it comes to saltwater aquariums. A hydrometer allows you to measure and record the salinity of the saltwater. The saltwater which is both in your aquarium and which is prepared for water changes.

Maintaining stability and excellent water conditions are one of the foundations for success. [Read more]