Ok your not at a complete loss..though the process may not be in time. There are companies that supply Fish stores that have products that will make your water brackish. It's a simple pour routine from the bottle as indicated and will turn your water brackish. Most people with Mollies don't put them in brackish water and find a slew of later health problems as a result. Always read about any fish or tank inhabitant before purchasing so you'll have time to get their new home set up as they need it. In this case I would recommend you return the fish to the store and either purchase marine salt or a brackwater solution then return once your set up. You want a Specific Gravity of 1.005 but 1.002 - 1.008 is acceptable. Also if you plan on adding plants RESEARCH!!! Most common aquaria plants will not accept Brackwater conditions.
