Ich is a parasite. It is not caused by water chemistry, stress or any of the other things that are commonly blamed. It is caused by the parasite being present in the tank with fish present. Once it is cured properly, it will only come back when it is introduced to the tank again. It may come in on new fish, a net used in an infected tank, decorations that are moved around or new plants. There is an information post available using a link in my signature area that will give all the details if you are really interested in ich.
Salt is promoted by many people for mollies but I find that mine thrive without it. They never thrive in soft water or low pH water but do fine in hard, high pH water. Sea salt mixes do tend to raise pH quite a bit so I can understand people thinking that it is necessary. The more common mollies are really amazing fish. They can do very well in any water from straight freshwater to straight sea water and anything in between. Be aware that there are about 20 different fish that go by the common name of molly. Not all of them can adapt to brackish or salty conditions but the most common ones can.
