We've used that "2 part" drain cleaner available everywhere. I think Draino even has a version. It's two different liquids in the same bottle but when you pour it in, the liquids blend and supposedly have a better end result. I've used it 3 or 4 times since we've started full timing. I use it in the vanity sink as well. Once I run clean water down the drains, I make sure to run lots of water through the Gray tank for a while just in case the chemicals may have an effect on the tank material.