Why is black molly sitting on bottom of the tank?

its might be because of your water quality,do a 50% water change,see if that helps.also try and keep that ammonia down.ammonia is fish waste and un-eaten fish food that has been left to rot at the bottom of the tank so always siphon your tank.or your fish might just be sick,treat it straight away if its got signs of illness,if you leave it to late then it will be harder to cure.