The whole salt thing is very much an old methods vs new methods debate Lioness.
The only fish which need salt in their water are true brackish or saltwater fish, there are other fish which can do better with it in, other fish where it makes no positive difference and others still where it will kill them.
So we’ll leave aside the brackish and saltwater fish as this isn’t really what the discussion is about.
You get fish like mollies which are found all over the world, some of them in fresh water some in brackish, the ones we get in shops today are usually a bit of a hybrid of all the different sorts. They’ve all been so massively cross bred to get the colour strains we get today that you can’t differentiate where most of them come from. It’s widely proclaimed that mollies need salty water, there is a lot of anecdotal evidence to back this up, plenty of fish keepers (myself included) have had problems keeping mollies in fresh water, they just get recurring diseases, pop eye and fin rot being some of the common ones. Plenty of people will be able to tell you that when they moved the fish into brackish conditions these diseases stopped. So it’s natural that the conclusion drawn is that they do need salt, it is a sensible practical way to avoid these things. However my understanding is that when sciency people have actually investigated this they have found that it’s not the salt itself that stops these things, it’s just what the salt does to the water i.e. makes it hard and alkaline. So while it seems that mollies do not necessarily need the salt, they do benefit from its effects. The apple snails will be the same, they actually need the alkaline water because it contains more calcium which helps keep the shells healthy……. So again don’t need salt, but benefit from the effects of it.
Now the other side to the story is salt as a magical cure all, back in the old days of fish keeping water changes were few and far between, as you can imagine nitrates were sky high in tanks that never got changed, what salt actually does is reduces the toxicity of nitrate to fish. So in these older tanks diseases caused by poor water, in particular by high nitrate were quite common. Adding salt helped to reduce the effects of the nitrates and as such the fish seemed to get better. Salt then got a reputation as being a magic cure all basically, now we know a bit more about aquariums this isn’t the case and it isn’t needed as a standard in most tanks.
The slight tangent of an angle though is that salt can actually cure some diseases particularly parasitic type things, it’s a fairly grim way of treating things but can work. What you do is mix up some saltwater as if you were preparing it for a marine/brackish tank, so using proper marine salt, and give the infected fish a short bath in it. Most fish will survive in the wrong salinity for a short time but not long. What it does is the parasite/bacteria can’t live in salt water, because its lifecycle is a lot shorter it is affected much more quickly so it dies off before the fish is severely affected. As you can imagine it’s easy to get wrong.
