It’s always a good idea to remove that nasty buildup from the bottom of your salt mixing container before making a new batch of saltwater. The gunk can consist of dried calcium deposits or even a dark dusty like residue, both of which can foul up your new water. To clean the container, try filling it with tap water and vinegar. This will remove any calcium buildup with ease. The other stuff might come off with the vinegar bath, but sometimes hobbyists need to get into the container with a hard brush and some bleach. Of course, if bleach is used, then the container needs to be rinsed thoroughly and dried out before being used again for mixing saltwater.