Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Using salt in the aquarium

Male Sailfin molly, Poecilia latipinna
Sailfin molly, Poecilia latipinna


There are more myths about the use of salt in the aquarium than almost anything else. Does it act as a tonic, should it be added routinely as a preventative medication or does it do more harm than good?

In the good old days

Going back to the days of the goldfish bowl before the nitrogen cycle was really understood by most fish keepers the fish kept in a bowl were probably subjected to high levels of nitrite on a fairly regular basis. Salt does counter the toxic effect of the nitrite and so fish which had salt added to their water generally survived longer than fish which were exposed to nitrite and left effectively untreated.
No one really knew why but adding salt seemed to help the fish stay healthy and so began the myth that adding salt helped the fish.

Salt uses

Salt does have its uses and it can be relied upon for the treatment of white spot, hydra, leeches and other parasites, nitrite poisoning to name but a few but it should not be added to a fresh water aquarium for any other reason. It is also worth noting that some treatments where salt is used it can actually make the condition worse. Salt causes the fish to make more mucus, some parasites like the Gyrodactylus fluke actually grew better on fish treated with the usual 3gm/l of salt and to remedy this the dose actually needs to be doubled.

Salt when used as a treatment may impact on the nitrifying bacteria in fresh water aquariums. So if possible use it in a separate vessel and not the main aquarium.

When added routinely It will place the fish under stress and it performs no useful function.

Some fish like mollies (not guppies, swordtails or platies) are really brackish water fish even though they are sold as community fish and they DO need some salt adding if they are to be at their best.

Do fish need salt in their water

True fresh water fish have evolved to live in fresh water and they have a different physiology from fish which have evolved in brackish or salt water if they are forced to live in conditions which don't suit them it could lead to problems and premature death.

Fish which come from soft acidic water have bodies which are designed to hold on to salts in their bodily fluids which contain more salt than the surrounding water. To achieve this their kidneys produce lots of very dilute urine so they are able to get rid of excess water and keep hold of the salt. Water is absorbed through osmosis because a weak solution will always try to dilute a strong solution. If they are placed in water which has a higher salt level than their bodies then water will leave their body through osmosis and they will effectively dehydrate. Although they can adapt and cope with a certain level of salt their bodies are being made to do something which they weren't meant to do.

The more salt there is in the water the more stress the fish will be under, and if the level of salt is high enough it will cause the eventual death of the fish.

Marine fish and fish from brackish water have bodies which work in an entirely different way. Their bodies are designed to hang on to water and expel salt, in order to do this they have to drink large amounts of water and they pass very small amounts of very concentrated urine in order to keep the salt levels of their bodily fluids less than that of the surrounding water. If such a fish (like a molly) is put in soft fresh water it could take on to much fluid and end up suffering from dropsy like symptoms.

Fish which have lived for hundreds of thousands of years in fresh water do not need salt adding to their water.

Which salt to use

Marine salt:
If you need to add salt for marine or brackish water fish then use marine salt because it contains all the buffers to raise the pH and vital trace elements but none of the other additives that salt intended for human consumption contains.

Tonic salt:
Is salt with no additives usually sold in fish stores. Often recommended as the only safe salt to use for both treating and even routinely adding to the water. Neither claim is true. It is just very expensive salt.

Table salt:
Fish stores will often tell of toxic additives which will harm your fish if you use this very cheap salt as opposed to the very expensive salt that they sell. This is a myth, salt intended for human consumption does not contain anything that will harm your fish when used properly for treating various ailments. Ordinary salt is perfectly safe to use for treating fish.

Main Facts


Iodised salt is dangerous to fish.

How often are we told this? Has there even been a study about this because despite a very intensive search I have been unable to find any scientific evidence to back up the claim.

We typically use salt as a remedy at a dose of 3g/litre.

Iodine is added to salt and makes up approximately 0.002% to 0.004% of the total. So we are actually adding 60 - 120 micrograms of iodine per litre of water.

1 microgram =1,000,000th of a gram.

Natural sea water typically contains 60 micrograms of iodine per litre, it is an essential trace element.

In short the amount of iodine added with iodised salt will not harm your fish.

Sodium ferrocyanide

It is obvious where the concerns lay with this additive - cyanide.

We all know how deadly cyanide is and so all compounds containing it must also be deadly right?

Wrong. compounds behave in very different ways to the elements from which they are formed.

Put a naked flame to hydrogen in the presence of oxygen and you'll make an explosion, but water is a compound of hydrogen and oxygen and that isn't explosive, quite the opposite in fact.

Sodium ferrocyanide is a none-toxic food additive (E535). It is a very stable compound even in water and is completely safe.

Iodised salt is a fraction of the price of salt sold for fishkeeping, it is more widely available and its effectiveness as a remedy is exactly the same as none iodised salt.

Marine salt

Should not be used as a treatment in fresh water fish because it contains buffers which will radically alter the waters pH value. But it should be used when keeping brackish water species.