Fishbabymom, if you are going to add salt to a tank to make it brackish, use ocean salt like they use for a saltwater aquarium. So called aquarium salt is only going to make the water saline, not brackish. The only use I know of for aquarium salt is for treating ich. Some mollies come from areas in the wild that are somewhat brackish and if you happen to have one of those, the sea salt would help them. Many livebearers can tolerate some salt for a while but it should not be used indiscriminately. Unless you know that your mollies require some salt, raise the water hardness and pH instead. All of the common livebearers do better in hard water with a pH from about 7.5 to even 8.0.