My bath room sink has been draining slower and slower. I took a peek down the drain and saw lots of black cruds. Possibly they are build ups from soap, tooth paste, and snot. I do not have this problem with my bath tub. When I replace my bath tub last year, I looked at the drain pipe and it was clean. When I replaced my sink down pipe last year, it has the same build up. A year has gone by, and it is starting to clog again. What can I use to clear this?