Every so often, I hear people say "I‘ve been told that I should add some salt to my aquarium." It is hard to find where this riginated, it seems to be "old wives tale", that has been around for quite some time. It's clear to see that in a some cases, this is motivated by the shop wanting to sell specific aquarium products that is basically packaged up kitchen salt or seasalt.
If we‘re being pedantic, in chemistry, a salt is any compound formed by mixing an alkaline substance with an acid. In this discussion, salt means sodium chloride, NaCl, the same thing we use to salt our food. With a few exceptions, the water our typical aquarium fish is very low in minerals, and even lower in sodium and chloride. The exceptions are the rift lakes, that has, relative to common freshwater, relatively high levels of sodium, but still very low levels of chloride. The main reason for this is that chloride tends to dissolve quickly, and eventually end up in one or another ocean.
It is almost never necessary to add salt for freshwater fish.
As I‘m already on a rant, let‘s make it clear that regular kitchen salt is just as good as any aquarium specific product. No need to worry about the additives. Overdosing to the point where the iodine or ferrocyanide are causing a problem will have killed even the most salt tolerant of marine fish several times over - it may even be required to raise the temperature to dissolve the salt. The exception in this is when keeping fish that are in brackish water, in which case true marine salt which contains a mixture of different kinds of salt, adding essential elements for fish that live in brackish water.
There are a few circumstances when salt is actually useful:
Short term, low to intermediate levels of salt isn‘t terrible. Just like a human eating a lot of salty food once every few weeks is not terrible. Naturally, like most substances, a too high level is harmful or even lethal even with short exposure - most freshwater fish will not survive more than a few hours in marine conditions, never mind feel "comfortable".
However, long exposure to higher than natural levels of salt will, like with humans, have detrimental effects on the health of the fish. Fish that live in water with very little minerals in the water have evolved to "keep" minerals, and not to keep them out. The opposite happens for fish that are evolved to live in high mineral water, such as Rift Lakes (such as Lake Malawi and Lake Tanganyika). However, even here, the fish live with very low levels of chloride in the water, so should not really have salt in the water. It is much better to use a mix of minerals that replicate the conditions in the real body of water that the fish comes from. This is acompletely different subject, so I‘ll return to that at some other point.
Ingredients: 100g contain 580 mg sodium perboricum, 97 mg potassium chloricum, 58 mg magnesium peroxide, sodium chloride ad 100.
So, contains 580 + 97 + 58 milligram of "not NaCL" compounds - in other words 99.265% "plain salt". The other compounds are:
- Sodium perborate, NaB03 an oxygen source. (sodium perboricum is a latin form which makes it sound fancier than the stuff you find in washing powder that has "active oxygen" or some such fancy description).
- Potassium chlorate, ClKO3 - used together with some other compounds to make the white smoke when a new pope has been chosen. Here it is used to generate oxygen.
- Magnesium peroxide MgO2. Another oxygen generating compound.
So it adds oxygen to the water.Yes, it really does. If we take a 100 liter aquarium with moderate levels of oxygen, say 6 ppm, or 5mg/liter of oxygen. If we then add, say, 1g of salt per liter, it will add a total of 7.35mg of oxygen producing compound per liter. Not all of that is oxygen. Out of the 100g salt added, we have 38mg from the potassium chlorate, 33mg from the magnesium peroxide and 340mg from the perborate. That makes a total of 400 mg for the tank, so raises the level to 10 ppm. Which is splendid. The only problem is that the oxygen level will preetty soon fall back down to whatever the circulation of the tank naturally provides. So, short term, great. Long term, not much effect, unless more salt is added, and that will eventualy lead to too much salt. Better to add more circulation.
ppm - Parts per million, or milligram per liter. 0.1% = 1000 ppm.
ppt - Parts per thousand, or gram per liter. 0.1% = 1 ppt.
 That would technically make it a brackish aquarium, although there are of course fish that live in water that is just marginally saline, but less than the level of 0.05% (0.5 ppt or 500 ppm).