It could be that they are getting too much water or there is extra watter sitting in the bottom of the pot. If the roots are sitting in constant wanter they will rot. Try doing a google search on the type of houseplant you have and you should be able to find out which kind of water and light they require. Some plants require direct sunlight, such as in a window. And, some require indirect sunlight such as sitting on a table in a lighted room. Good Luck.