Salinity is a measurement of salt dissolved in aquarium water

Its important that you maintain a fairly stable salinity reading for the health of your tank inhabitants. Fluctuations in salinity will stress out your tank inhabitants and can weaken them and increase the chances of infection.

Salinity actually measures more than just the amount of Sodium chloride (table salt) in the water. There are other salts that are in solution too (salts of Magnesium, Calcium, Strontium, etc) That's one reason why you cant just add common table salt and make saltwater for your aquarium. Saltwater actually contains at least a little bit of just about all stable compounds including gold!

The most common way to measure salt is to actually measure the Specific Gravity of the saltwater. This measures the density of dissolved solids in the water, which closely corresponds with the amount of salt in the water.

All you really need to know is, you need to buy an inexpensive salinity measuring device called a hydrometer. They are typically around $15 or so. But be warned they can be very inaccurate, as much as 5 ppt in our experience, especially as they age and get a coating on the floating needle. You might compare several readings of the when buying them in person, to weed out the ones which have extreme readings.

Or you can buy a more accurate device, called a refractomer (pictured at left) . These are around $50 or so online. Refractometers optically measure the amount of dissolved material in the water.

For corals, we like to keep our salinity (specific gravity) at around 1.026. For fish, a lower SG of around 1.023 seems to be more beneficial. You can keep them together and we recommend an average of about 1.025 for the two.

The most important concept when dealing with salinity, is try to keep it as stable as possible. Be sure to add filtered FRESH RO Water, not SALTWATER, to makeup for any evaporation losses. If you don't, then your salinity will continue to go up as your water evaporates.