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Salt in Freshwater

Question: I have been advised to add salt to my freshwater, is this a joke?

Answer: No, not at all, Many fish keepers add salt at the rate of 0.5 - 1 teaspoon per gallon as a general tonic.

"Freshwater" is very much a relative term. All water contains some salts, how much determines wither we call it "Fresh", "Brackish" "Saline" or "Brine".

Water salinity

Fresh water
< 0.05 %
< 500 ppm

Brackish water
0.05 - 3 %
500 - 30 000 ppm

Saline water
3 - 5 %
30 000 - 50 000 ppm

Brine
> 5 %
> 50 000 ppm

Enough of the technical stuff, lets look at how fish react to salt.

Fish, just like humans, have salt and other minerals in their body tissue which they must maintain at very precise levels if they are to stay healthy. How a fish regulates these level depends upon its normal environment. In Freshwater their bodies have a greater concentration of salt than their surroundings, so they tend to absorb water which they then excrete through their Kidneys. In this respect, the less salts in the water, the harder their bodies have to work to control their internal levels.

True, this is part of their genetic makeup and is based upon their natural environment. But we must consider that few fish come from our version of "Fresh Water", i.e. the stuff that comes out of our water taps. In nature most "Fresh Water has much higher levels of salt then our drinking water. What's more, not all fish are truly Fresh Water, for many, home is somewhere in between!

Popular with Aquarians is the Cyprinidae (Carp) Family which includes such favourites as the humble goldfish and their more exotic cousins, the Koi, both species noted for their need of salt. Indeed many Koi keepers maintain a minimum salt level of 0.03%, which they may even increase to 0.06% for short periods as a tonic for stressed or ailing fish.

Hexanematichthys seemanni)Another species often seen in aquatic stores is the Colombian Sharkcat (Hexanematichthys seemanni), and sold as a freshwater fish. It is true that young specimens can be kept in fresh water for a time, but ever increasing levels of salt are necessary for it's long term survival. When adult these fish need very brackish, even Sea water to survive..

There are some species, such as the Clown Loach for example, which do not like a lot of salt in the water, but even they will be quite happy at the very low levels used by most fish keepers. It is also worth checking on your live plants, some tolerate a lot of salt, others do not.

Maroon Clownfish in an Anemone