Last year I bought a home on Padre Island in Corpus Chrisit, TX that was built in 2000. I am having a problem that I've never seen before. The sinks in both bathrooms continue to grow some kind of black mold-looking sticky substance that grows all the way to the stopper. I clean the stopper and the drain (as far as I can reach with a toothbrush) with Comet and then bleach about once a week. Nothing works. Has anyone else ever heard about this and know what I need to do?