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Last Updated: 8/4/10

On 7/14/07, I moved my salt information to this page so it is all in one place with some additional information.

Types of Salt
Uses for Salt
Animals and Plants that are Sensitive to Salt
Salt Doses and Calculations

On my aquarium frog page is a section on frogs and salt.

Go to the fish health page.


Salt can be added to tanks to decrease the fishes' osmotic stress and reduce the population of pathogens. In other words, it can kill some bad bacteria and parasites. Be sure to use salt that is specifically for aquarium use as those with additives like iodine can harm or kill fish. Because fish have high salt contents inside their bodies, water flows into their bodies because, by nature, the concentration both inside and outside the body wants to be the same (equilibrate). To prevent this, the fish pumps water back out which takes energy. By adding a little bit of salt to the water, some fish (not all) benefit by expending less energy. Fish that prefer virtually no salt include discus, catfish, and other soft water fish. There is always some salt in water unless it is distilled or deionized. No fish can survive in 100% distilled or deionized water.

Types of Salt

The term "salt" refers to anything that is a combination of two ionic (an anion and a cation pair up) inorganic chemicals. When most people say "salt," they mean sodium chloride or NaCl which is common table salt. Other common salts include magnesium sulfate (Epsom salts), calcium chloride, and the list goes on.

"Table salt" often contains more than just sodium chloride. They often add iodine to increase the iodine intake for humans. Iodine is needed in low amounts for all animals. Shrimp are high in iodine so iodine is sometimes added to tanks with shrimp. Fish do not like a lot of iodine. Too much iodine harms the thyroid gland. Because it is hard to regulate how much they are getting, it is better not to use table salt for fish aquariums. In addition to the iodine, table salt often has anti-caking agents which are not good to add to the tank either. Home improvement stores and pool stores may sell salt. If it says it contains only sodium chloride, it can be used for fish aquariums and ponds.

To add to the confusion, some salts sold for freshwater aquariums are just sodium chloride while others are artificial sea water mixes and others are actual evaporated sea water. For freshwater animals, the just sodium chloride is fine but, if you use the salt water mixes (which contain other salts and minerals and trace elements), that should be fine as well. Something like Instant Ocean sold to make up marine water is going to more expensive but should not be harmful if used to add salt to a freshwater aquarium in low levels.

To be safe, it is best to buy salt designated as aquarium or pond salt. Then, you know it is safe for your aquatic friends.

Once you add salt, you may need to test the water to see if you have gotten the amount of salt that you need/want.

Here are links to aquarium salt, pond salt, a salt test kit (they stopped carrying it?), and a new pond hydrometer respectively sold at Drs. Foster and Smith.

Uses for Salt

Salt is added to freshwater aquariums for these reasons:

Fish who prefer some salt in their water

There is a small class of fish that are classified as brackish which means they are in between freshwater and saltwater. Examples include freshwater puffers, scats, monos, archers, and more. They need to have salt added to their water.

Many livebearers prefer salt and some will not do well without it. Mollies can live in brackish water and are happy with a tablespoon per gallon or even more of aquarium salt. Guppies also like some salt, perhaps a tablespoon per few gallons.

Reducing the toxicity of nitrite

This is extracted from my water chemistry page.

Adding salt, at least a tablespoon per 5 gallons, will help detoxify nitrite if that is the problem. Keep in mind that salt is harmful to plants and some sensitive fish in high doses. Once nitrate test kits register nitrate present, the danger has most likely passed. This takes about a month or so.

Also, note that salt should be used sparingly if zeolite is also being used. This is because zeolite can be refreshed using sodium salts which replace the ammonia in the zeolite. If lots of salt is added to a tank or pond with zeolite that is full of ammonia, the zeolite may release noticeable amounts of ammonia which could be harmful. Use the zeolite to remove ammonia and later, if nitrite becomes too high, use salt to render it less toxic and remove the zeolite. Using small amounts of salt in a tank or pond with zeolite should not make much difference.

Regulating osmotic pressure

Sodium chloride (table salt, aquarium salt, pond salt) increases the ion concentration in the water. Inside the fish, there are more ions than in the pond water. When you have two bodies of water next to each other, they want to reach equilibrium which means water will flow from the higher concentration of ions side to the lower concentration side. For the fish, that means that they must constantly pump water out of their bodies that is flooding into it. That takes work. When we add a little bit of salt to the water, it reduces the work the aquatic animals have to do. They save energy. At some point, the salt gets too high and kills the animals as then there is more salt inside their bodies than in the water, and the water flows out of their bodies, and they dessicate.

Deterring parasites, bacteria, and funguses

If you look at nature, freshwater ponds normally do not have much salt in them. They also do not have many fish and thus have low amounts of fish nasties which include fish parasites, bacteria, funguses, and viruses. Now, in our aquariums and ornamental ponds, we usually cannot help ourselves, and we put a ton of fish in. Due to higher concentrations, any fish nasties also multiple happily.

Just like with the fish (see above), the fish nasties have salt inside their bodies. When salt is added to the water, they have less water flooding into their bodies. If there is enough salt, water starts to go out of their bodies, and they dessicate (dry up). The fish nasties are more prone to that happening than the fish which means that salt will kill many parasites, bacteria, and funguses.

For treating dropsy, it is often suggested to use magnesium sulfate (Epsom salts) instead of sodium chloride. The dose is 1/8 of a teaspoon per 5 gallons for Epsom salts according to one web site and a tablespoon per 5 gallons according to another!

Animals and Plants that are Sensitive to Salt

Animals that are sensitive to high levels of salt include scaleless fish (including catfish), snails, shrimp, frogs, insect larvae, microorganisms, and even good bacteria. They may all die before the salt concentration is toxic to most fish.

Then, there are the plants. Plants do not like salt at all. Too much salt, and they will wilt.

So, it is best to add just the amount of salt that helps the fish a little bit, deters the fish nasties, and does not harm the sensitive animals and plants. The recommended doses on the pond salt containers may technically be safe for say a healthy goldfish but are they safe for the snails, sensitive fish like orfe, sick fish, frogs, and plants? So, I play it safe and use half as much. If you used no salt, the pond would probably do just fine. If you have soft water like me, then there are fewer natural ions and salts so adding salt becomes more important.

On my aquarium frog page is a section on frogs and salt.

Salt Doses and Calculations

As a general treatment or preventative, salt can be added to tanks at up to one tablespoon per 5 gallons if the fish are not sensitive to salt. I use a tablespoon of aquarium salt per 5 gallons in all my tanks. That is low enough to not bother plants, snails, scaleless fish, or sensitive fish. Of course, brackish water fish, livebearers, and saltwater fish would require more salt; and soft water fish like discus and catfish would require less. If the fish have an outbreak of ick, other small parasites, fungus, or unknown problems; adding more than a tablespoon per 5 gallons may improve the situation (a tablespoon per gallon for fish that can take it, a tablespoon per 2, 3, 4, or 5 gallons for more sensitive fish).

As a preventative of parasitic or other outbreaks in ponds, a 0.1% salt concentration in ponds is best according to experts. That level can be maintained in koi-only ponds. That amount is equal to 0.75 pounds of salt per 100 gallons. See below for a sample calculation to determine how much salt to add to ponds. Plants do not like 0.1% salt; it is too much. Amphibians (frogs, salamanders, toads, and newts) do not like that much either. For ponds with plants and other animals, try to stay below 0.05% pond salt. I probably have much less than that. I only add salt to my pond in the early spring. Rain (and water changes) will wash out salt in a pond over time. My pond salt test kit says to maintain 0.2% for ponds without plants and 0.1% for ponds with plants but I think half those amounts are better for reasons previously stated.

For sick large fish like koi with fin rot or parasites, a short (30 second) dip in a salt solution (about a cup per 10-20 gallons) may improve their health. Remove the fish immediately to its tank or pond if it jumps or rolls over since it could die in a salt dip. Salt dips can kill parasites and other organisms by creating such a difference in salt concentration inside their cells versus outside that they literally explode.

Example Calculation:

I want my pond to have 0.1% salt concentration in it. The pond is 5 feet wide by 10 feet long by 3 feet deep. How much salt do I add?

A 0.1% solution means that for every 100 grams of water, there should be 0.1 grams of salt. Since 1 g water takes up 1 mL, that means 0.1 g salt per 100 mL water or per 0.1 L of water. Since 1 gallon of water is 3.785 L, then 1 L is 0.2642 gallons. Thus, you need 0.1 g of salt per 0.0264 gallons or 1 g of salt per 0.264 gallons. Since 1 cubic foot of water is 7.479 gallons, then 0.264 gallons is 0.0353 cubic feet. Thus, you need 0.1 g of salt per 0.0353 cubic feet which is the same as 28.3 g per cubic foot. Your pond is 5'x10'x3' or 150 cubic feet. Thus, you need 28.3x150 or 4250 g of salt for your pond. Since one pound of salt is 497.7 grams, then this is equal to 8.5 pounds of salt.

Note that you cannot convert this into cups of salt to add because it depends on whether the salt is powder, little crystals, or big crystals as to how much room it takes up. In many cases, the container of salt you use has both a volume and a weight on it. For example, if the container is 10 pounds in 1 gallon, then you need 0.85 gallons or 13.6 cups (0.85*16) which is also 13 cups and almost 10 Tablespoons. By the way, your pond is 1121.85 gallons (150 x 7.479). Mathematically, this is all shown below.

0.1 g salt/100 g water x 1 g water/1 mL water x 1000 mL/ 1 L x 3.785 L/1 gal x

7.479 gal/1 ft3 x 5 ft x 10 ft x 3 ft x 1 lb/497.7 g = 8.5 lb of salt

I don't quite understand the calculations above. Can you make it easier for me to understand in summary?:
In order to get a 0.1% solution of salt in a salt-less pond, you need to add 0.75 pounds of salt per 100 gallons of pond water.

This web site also has a handy table for determining the amount of salt to add to a koi pond or tank in order to get a certain percentage of sale.

I read this interesting article in my college magazine on 1/26/10 about salt and its effects on amphibians and other animals in a pond:
The Science of Salt

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