tonic salt, natural rock salt [from supermarket], coarse salt and pond salt are pretty much all the same with pond salt been the cheapest.
marine salt is different as its like an concentrate of salt [man made not natural] with added nutrients and what not. don't know if its a good idea to put it in fresh water though but it does work in theory as full strength salt water is 30-50% of dissolved salts in parts per thousand, brackish is 10-30% and freshwater is only 0.5% or less.
sooo....using red sea salt as an example 1kg mix 30Litre so u would be safe if u add less then 5gram per 30litres that's not including the salt already in the freshwater so to be extra safe i would just put around 1 gram per 30litre of cause that's in theory so who wants to be the first :lol: :lol: :lol: