Aquarium salt is not equal to the sea salt.
Some people using sea salt instead of aquarium sea salt.
Salt-water animals needs the minerals and vitamins that there natural water would have. Sea salt will
eventually make your fish sick, because it doesn’t support the minerals and vitamins it needs. But
with aquarium salt, its designed to support the natural needs of regular sea ocean water.
It's best to use marine aquarium salt mix, not table salt.
The reason is that the marine aquarium mix contains buffers and mineral supplements that will
make a good water chemistry for brackish-water fish.
Sea salt for food use is almost 100% NaCl(what most of us commonly call "salt", without the buffers
and with only tiny traces of other minerals.
How much do you put in?
That depends on which brackish-water animals you are keeping.
The definition of "brackish water" is just "water with salt in it," so it can mean anything from water
that is slightly saltier than pure fresh water to full-salinity ocean water, or even hypersaline water
(water saltier than the sea). Brine shrimp are a familiar example of animals that live in hypersaline
springs and lakes.
So you should find out the recommended salinity for the particular species you have. Then you will
need an instrument to measure the salinity, generally a hydrometer or a refractometer. You add salt
a little at a time until you get the salinity you want.