Specific Gravity

 In layman’s terms, we can say that specific gravity (SPG) is a measurement of dissolved salts in water.

To mimic their natural environment — and depending on species — the inhabitants in our saltwater aquariums require a specific gravity of between 1.022 and 1.025.

specific gravity for coral reef aquariums

Coral reef aquariums require a specific gravity of 1.025

Specific Gravity For Fish-Only And FOWLR Aquariums

Many aquarists who keep fish-only or fish-only-with-live-rock (FOWLR) systems choose to go with the lower end of the spectrum, sometimes as low as 1.020, because microbes and disease-causing parasites do not survive well at low salinity.

Lower salinities literally cause these microorganisms to rupture and ‘explode’ because of their inability to handle osmotic change in salinities outside their survivable range, which is why freshwater baths are sometimes used to kill parasites that cause marine ich (Cryptocaryon irritans), commonly known as white spot.

And because fish-only and FOWLR keepers tend to keep larger fish species (and at higher populations), lower salinities help increase dissolved oxygen levels in the aquarium, reducing stress on the inhabitants while encouraging natural behaviors.

As a general rule, I would recommend a specific gravity of 1.022 for fish-only and FOWLR aquariums. If disease like ich is suspected, freshwater (reverse osmosis or deionized) can be added to the tank, to lower the specific gravity temporarily for several days, to 1.019 or 1.020.

Specific Gravity For Coral Reef Aquariums

The specific gravity of natural seawater around coral reefs is typically 1.026.

For this reason, it is advisable to maintain the specific gravity at not less than 1.024 in a full-blown coral reef aquarium, with 1.025 or 1.026 being preferable.

Clams, too, need the specific gravity to be at natural seawater levels in order to survive.

Specific Gravity For Invertebrates

If you intend to keep shrimp, snails and other invertebrates, specific gravity should be maintained at not less than 1.022 and not more than 1.026.

Measuring Specific Gravity | Refractometer Or Plastic Hydrometer?

A refractometer, regularly calibrated with distilled water for accuracy, is one of the best ways to measure specific gravity if you need a very precise reading.

But for routine water changes, for example, I find my good ol’ Coralife swing arm plastic hydrometer to be accurate enough for checking the specific gravity of a newly made batch of saltwater. Since only 10% to 15% of the total volume of water will be changed, it puts me right in the ballpark.

The main reason why people find their plastic hydrometers inaccurate is that they are either using an inferior no-name model, or they don’t rinse out the hydrometer thoroughly under a running tap after every use. Salt deposits that encrust on the plastic swing arm can affect readings greatly!

Fluctuations In Specific Gravity

While most of our marine inhabitants can handle fluctuations in specific gravity over the course of several days, we should avoid fluctuations of more than two-thousandths in a 24-hour period. We should avoid, for example, a fluctuation of 1.026 to 1.024 or vice versa.

Smaller tanks, especially nano tanks of 20 gallons or less are more prone to fluctuation in salinity due to evaporation.

This is especially true for small tanks that are cooled by cooling fans. When evaporation is high and not replaced promptly with freshwater, specific gravity rises since salt does not evaporate.

Using An Auto Top-Off To Keep Specific Gravity Constant

Many aquarists, especially those with delicate corals, have an auto top-off system. When the system senses that water levels have dropped to a certain level, a small pump sitting in a separate tub of freshwater is activated. Freshwater is pumped into the system until another sensor deactivates the pump when the desired water level has been reached.

In addition to keeping specific gravity in our aquariums constant, having an auto top-off system is a great convenience — fill the storage reservoir with freshwater once a week or so and let the auto top-off take care of the rest. No more filling pitchers and manually pouring freshwater into the tank or sump every few days!

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