A Chrysanthemum plant is really an outdoor plant that has become a popular floral gift and flowering plant for home and office. It is second only to the rose as the most popular cut flower in the world. Mums are members of the Asteraceae Family which includes over 20,000 flowering species. Chrysanthemums were brought to Japan by Buddhist Monks in 400 AD and have remained a very important part of Japanese culture to this day. Mums last 3-4 weeks indoors with very little care and in almost any environment before needing to be replaced. The blooms which come in various shapes and colors can be daisy-like, pom-poms, or buttons, and may be yellow, burgundy, pink, or white. A Chrysanthemum is considered to be a slightly poisonous plant and should be kept way from small children and pets. Read more about common houseplants that can be dangerous in my book Don’t Feed Me to Your Cat: A Guide to Poisonous Houseplants NASA lists the Chrysanthemum as a plant that can cleans the air of benzene, formaldehyde, and carbon monoxide. You may also enjoy sipping Chrysanthemum tea to relax.
It sounds like you have Fungus Gnats. Do not allow your plant to sit in water or the soil to be too wet. Both of these will breed gnats. You can spray with the green solution, but be careful to avoid the flowers on your plant; aim for the soil.
You’re over-watering your plant. Allow the top few inches of soil to dry out and even allow the leaves droop a little before watering it.
According to NASA mums can rid the air of benzene, formaldehyde, and carbon, monoxide.
I think the leaves of your plant probably got too cold and damp and developed botrytis. Cut off the diseased leaves and move your mum to a warmer brighter location.
Nothing! Florist mums, compared to garden mums, are disposable indoor houseplants. Their blooms only last 4-6 weeks and they don’t survive when planted outdoors.