For one Molly you will need at least a 10 gallon tank with medium hard water, pH around 7.2. Temp around 75F. adding 1 level tsp aquarium salt per gallon of water will also be good for them.
That is my favorite question. You see, tap water from a faucet has CO2 in it, but has a little nitrogen monoxide, if not very clean. Tap water also has lead in it, unhealthy for plants and people, so...