I was in the same situation,,I already had house plants, 3 different types of Ivy, a Yucca, and tree palm. I was given a Peace lily from a friend. 1 week later, little gnats flying around. I removed 2 inches of the potting mix from ALL the indoor plants and topped them off with sterile sand, topped with decorative rocks. Gnats can't get thru the sand nor can the larvae. I thought I won the fight, but my containers are the "self watering" type. gnats just flew in where you water from the bottom. Taped all openings and let the plant dry out, so to speak. The best thing is to break the breeding cycle..if the gnats can't get in, no eggs,,,and the sand prevents new ones from getting out. I also used sticky traps around the plants and on the windows..caught around 100+. That's the adults. After about a month, changing traps etc, I'm just about gnat free. 100, 40, 10...So far so good.

BTi is the best thing I have ever found for fungus gnats. You mix it with water, then water your plants with it. You have to do it several times to kill all of the larvae in their various stages. Yellow sticky traps will take care of the adults. If you can't find BTi in a liquid, you can buy a package of Mosquito Dunks. Put one of these in a gallon of water overnight, then remove it and use the water on your plants. You can reuse the leftover Mosquito Dunk for your next batch. Do this several times a week for 3 weeks. You can also buy Mosquito Bits (or crumble up a Mosquito Dunk) and put it on top of your soil. BTi is harmless to humans and pets.

pjames might be on to something. I have collected manure from different piles. I always do a test of the manure before I feed it to the worms, but I only do one test, then assume the whole batch is good, even if the manure may have come from different horses.. OTOH, the die off seems to continue until long after I have fed horse manure. There is no clear consensus on whether dewormer is actually harmful to the worms. That is something I might experiment with when I have a healthy worm population. I would like to get a definitive answer on the dewormer question. I was reading an article yesterday (I can't find it now) that says that dewormer is 95% broken down in the horse's system, and degrades completely with exposure to sunlight. If the problem is dewormer, that would explain why I am only having a partial die off.

A picture would have been helpful to see the extent of the rot. The rotted areas need to be eliminated completely and cleaned. Ficus can take a lot of abuse. Then treat it like a cutting. Moist, well drained soil with a means to keep it humid is ideal. An empty large plastic coke bottle with the bottom cut off works as a good humidity dome for small plants. If you are going to use a bottle wash it with diluted bleach first and keep the cap off. Take the bottle off every day for a bit to air it out. The plant may need to be anchored to the soil so that it does not move when it is putting out new roots. Reduce the leaves to may be 3-4. Put it under CFL light. Not incandescents - they produce more heat than light.