Do you know anyone else with a fish tank? Ask them for a little gravel, or part of their filter and add it to your filter. That will expedite the bacterial growth you need in your tank. Fish waste is ammonia, which is toxic for fish. Bacteria convert that to nitrite (also toxic to fish), then finally nitrate which isn't nearly as toxic to fish. It is still toxic in large quantities, so that is why you need to do water changes frequently. Getting this bacteria in the tank is what is called "cycling" the tank.
If you just throw fish in there without cycling you tank, the ammonia will build up and eventually start burning the gill of the fish, causing them to eventually suffocate and die. The bacteria needed will eventually grow, since it's found naturally in your tap water, but not quick enough where you don't kill some fish in the process. Molly's are extremely tough and will most likely live through this process so they are used sometimes to cycle a tank, but it's still not good for them and will severely weaken them.
I had a 46 gallon bow front tank for a long time, we recently moved though so I disassembled it and don't have any plans to put it back up. My grandpa and dad were both very into fish though, so if you have any questions or anything feel free to ask.