Just keep testing daily and when the ammonia or nitrites rise again, do a water change. You might save the Molly yet.
Good luck! It's a bit of work but it can be done.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.