First off, stop fertilizing the plants! Plants can get seriously damaged from being over fertilized. All Miracle Grow soils have tiny fertilizer capsules in it designed to fertilize plants for at least 3 months (that is one of the reasons I don't use Miracle Grow). Adding additional fertilizer is just causing damage to the plants. Fertilizer wont cure a unhealthy plant.
The moisture control potting soil is another problem. The soil stays too wet for too long and causes the plants to develop root rot. If the plants already have root rot, then they only way to save the plants will be to cut off all the stems and reroot them. You can not save the plants with no leaves and the Polka Dot plant probably won't reroot. After the stems reroot, plant them in just regular potting soil. If you use Miracle Grow potting soil, do NOT fertilize the plants for at least 6-8 months.
Chemical insecticides are rarely useful on houseplants and are often toxic to humans and animals. The best way to get rid of fungus gnats is small dishes of soapy water and a potato. Place a couple of small bowls of water with a few drops of dish washing liquid mixed in. That will attract the adult gnats and drown them. Change the soapy water in the bowls every day. Fungus gnats lay their eggs in the soil. The gnat larvae are extremely hard to see in the soil, they are small, almost transparent worms. To get rid of the gnat larvae, peel and slice a potato. Stick the potato slices into the soil. The gnat larvae will be attracted to the potato slices and will burrow into them. Simply remove the potato slices and throw them away. Put fresh potato slices in the soil every day until you no longer find any fungus gnats. Fungus gnats are really annoying but they don't really damage houseplants.
