Aquarium salt works best, I've heard & used when my 75g had ich. Black mollies aren't brackish, although they can be kept in brackish. Sailfin mollies are the only true brackish mollies I know of.
New fish and plants can bring diseases & parasites into your tank, along with rocks and driftwood that has been in somebody else's tank. How can you stop this? Quarantine all fish for 2-3 weeks before adding them to your main tank so you can see if they display any illness. If you aren't sure about plants you get, it's best to QT them for a few days.