In the upstairs bath I have a long counter with double sinks. Both sinks had standing water, so I used a bottle of drain opener in them. That worked for a couple of weeks, now they are backing up again. But the curious part was that I plunged them, and a few hours later the water went down. Then the next day, the water was back; and no one has been using them! I am about to go buy some more drain opener, but this stuff is really hard on the cultured marble. Any ideas?