I have two adult mollies and three, four month old baby mollies. Yesterday one of the adult mollies died suddenly and now the other adult Mollie has become very aggressive toward the babies and I'm not sure what to do about this. Any ideas ?
Fish deaths are typically water related. This is especially true for Mollies. These fish are the most sensitive to changes in water conditions and their diet than any of the other livebearing fish.
You can deal with bullying by keeping the surface of the tank well planted. Water sprite, Hornwort and Anacharis are some of the fastest growing floating plants available. Drop some individual stems of any of the these into the tank. They'll grow without being planted in the bottom material and provide hiding places for the fry. Surface plants also help the adults feel more comfortable in their surroundings. The plants will eventually cover the entire surface of the tank and need to be thinned regularly.
Mollies are jerks. Plain and simple. They are also quite large and powerful fish by small Freshwater community tank standards, so their tank mates stand little chance. I'm part of the pufferfish community online and mollies have been known to bully puffers of the same or larger size. If such a small fish can bully a puffer, that's hardcore. Truly, people do not realize how aggressive mollies are. They have a cute name and they can be so charming to anyone who walks up to their tank, but don't let it fool you!
Also, mollies are poopy poopers who poop. These fish do not belong in anything smaller than a 30 gallon.
Re-home immediately. If you are really attached to mollies, you'll need to get a bigger tank and even then I'd only have one molly in a tank unless it was very large and heavily planted.