Sailfin mollies have been found several miles out to sea. Mollies are essentially fresh to brackish fish -- ime they typically do better with at least some (ocean mix) salt in the water but many will thrive in totally fresh. Some saltwater enthusiasts cycle their tank with mollies instead of damsels (another common fish-in cycle species for sw tanks).
Fishy? could you tell us more about your tank? Size, age, maintanence, stocking level, water parameters.....