Salinity

Salinity indicates the total concentration of salts dissolved in water. It is measured in PSU, ppt or by specific gravity (SG).

Detailed explanation

Salinity is probably the most important parameter for marine aquariums. Too high is stressful. Too low compromises the osmotic balance of corals and invertebrates. Rapid swings are worse than a slightly out-of-range value.

Sea salt is prepared by dissolving commercial mixes in reverse osmosis (RO) water. Each brand has different concentrations of trace elements. Some are formulated for reefs (more calcium and magnesium), others for fish only (less expensive).

A common pitfall: salinity rises with evaporation. The water evaporates but the salt remains. For this reason, topping up must be done with RO fresh water, never with salt water. Many beginners get this step wrong.

In brackish aquariums, salinity depends on the habitat to be reproduced. Mangroves live in 5-15 PSU. Archerfish do well at 10-20 PSU. Fiddler crabs prefer 15-25 PSU. There is no single value.

For freshwater aquariums the salinity must obviously be zero, but the TDS (total dissolved solids) is the equivalent parameter to monitor.

Reference values by aquarium type

How it is measured

Optical refractometer (the most used, economical and precise). Electronic conductivity metre (more convenient but requires calibration). Float hydrometer (inaccurate, not recommended). The refractometer must be calibrated with reference solution, not with distilled water.

What happens when it is too low

Corals that close, fabric that retracts. Fish increase mucus production. Invertebrates under osmotic stress.

What happens when it is too high

Reverse osmotic stress. Fish drink more water to compensate. The gills get irritated. In severe cases, corals expel zooxanthellae (osmotic bleaching).

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