First, before using any pesticide get a proper diagnosis because the pesticide may have unintended consequences, like taking aspirin for the pain caused by a bleeding ulcer. Many people think that because water runs out the bottom of a potted plant that the soil is totally wet. Nothing is further that the truth. Most, if not all, are made of dried shredded plant material which does not absorb water readily. Thing of a dry sponge, it does not absorb water when first place on it so you "force" the water in.

So how do you accomplish this feat with soil in a pot, simple. Take the pot, set in the sink, stopper the sink, fill with water to 7/8 the height of pot, let soak until the surface is moist (2-4 hours), drain the water and allow the pot to drain. Now you know the soil ball is holding all the water it can.

The garden centers and catalogs are all selling an inexpensive little "meter" that you can stick in the pot and it tells you how much moisture is there. I'm amazed to find that with twelve houseplants, all watered at the same time, all in the same potting soil = they will register differently at root level by the middle of the week. Also agree with petzold, try to never use any chemical or fertilizer as an "answer" until you know what the problem is. Right now it probably is in shock from all the changes and needs some rest. Did you change its location shortly before the leaf changes occurred? I had a peace lily do the same for "no reason" and it was the location. (The direction of the sun changed when winter came.)