I would like for this to be an open debate abour whether or not mollies should have salt in their water. Some people say you should, while others say you shouldn't, and i want it to get settled once and for all becuase that box of aquarium salt at the back of my cupboard is doing no good just sitting there...
i intitially had 4 black mollies in a pure fresh water set up. they seemed to be alright but started to lose some of their color. i then heard that u should have some aquarium salt in the tank with mollies and the like so i added a very small amount. this didn't seem to help at all. they all kept looking less and less colorful. after more research i discovered that a lot of people recommed using instant ocean or the like because it actually creates a more natural brackish enviroment due ingredients other then salt. after a few weeks they all began to look a lot more colorful.
my vote then is for a small amount of instant ocean mix to keep them looking their best.
I've never kept mollies (although they are on my short list once I can get another tank). Having read up on them though, everything I've seen (including the fish database on this site) calls them freshwater fish that thrive in slightly brackish water.
Mollys come from an environment (estuaries and near-ocean wetlands) where the amount of salt in there water fluctuates with the seasons. This does not mean that they thrive on changing salt conditions. It simply means that they can cope with changing fresh to brackish water conditions. These brackish environments do not only contain NaCl (table salt), they are a complex mixture of different chemicals. So, molly can live in a well maintained brackish environment. However, it is debatable if this is the ideal set up. First of all if you wish to keep them with other freshwater fish, you should not regularly add salt. It will irritate the other fish. Second, constantly maintaining a certain level of salt in your tank can create salt-resistant strains of disease (like ich). This can later make it very difficult to treat fish for disease. These are only a couple of things to consider, but there are more as well. If you personally know the breeder of your mollys you may want to house them in the small water conditions they were born too.
Personally I do not like maintaining mollys with salt.
PS: Many fish store suggest salt products just so they can sell them to you.
If you want to keep mollies in brackish or even marine that is fine. they can thrive in all types of water. Do they need it? No. if you buy them fresh then changing to a brackish or marine tank will take time, do it slowly. Will Doc Wellfish Salt work? No. You need a marine mix of salt and not dried out sea salt.
Purely anecdotal experience on my part -- my mollies do best with some salt. Always wasted away without it. I always used some marine salt mix, though.
