Rebecca L. from South Carolina asked if she should add salt in her freshwater tanks because she was told it prevents diseases and, if so, how much?

Here is the answer to Rebecca’s question: In many cases, adding a little salt to a freshwater aquarium can help, as it reduces the stress on the fish by assisting the fish’s osmoregulation (sorry for the technical term – what it means is it makes it easier for the fish to maintain itself physiologically in the water). But it all depends on the fish, and it will not prevent diseases in all cases.

Also, salt can be toxic to some fish, even in small doses. For instance, South America Tetras, Corydoras catfish and other fish from this region do not tolerate salt at all.

Also, you have to make sure to do water changes and not just keep on adding salt to the water, as soon your freshwater tank will be a saltwater tank.  I know this is obvious, but I have had even stores complain about freshwater fish dying, only to find out they are adding salt and not changing enough water, so just a “heads-up.”

Adding salt also depends on where you live. If you live in the Pacific Northwest, which has very soft water, then adding some salt (I would suggest cichlid salts – not sea salts) is definitely a good idea. If you live in Southern California, then adding salt is not going to help as much (assuming you do not have a water softener). But in general, a little salt is good for most freshwater fish.

Let’s assume you are going to add salt – so how much?  For most fish, you want about 0.3% or 3 grams per liter or 12 grams per gallon of aquarium water.

For comparison, seawater, for example, has on average 32 to 34 grams of salt per liter (128 to 136 grams per gallons) of water.

A LEVEL half-teaspoon of normal aquarium salt weighs 2.5 grams.  So the dose would be just over a half a teaspoon per LITER or 2 level teaspoons per GALLON (there are 3.875 liters in one gallon) but I am rounding to 4.

Adding salt can be a good thing, but add it slowly, say half the needed dose one day and half 48 hours later, and make sure to change the water before adding more.