Here lately, I've just been pouring white vinegar in there and either letting it sit for a while before scrubbing with the brush or scrubbing it when I first put it in there & then let it sit until someone uses it. Works pretty well!
This is what we use for most cleaning (initially used as part of my daughter's 6th grade science fair project). It is cheap (my favorite) and non toxic.
Â¼ cup white vinegar
Â¾ cup warm tap water
1 tablespoon baking soda
10 drops of tea tree oil (antibacterial, antifungal, etc.)
2 drops oil of lavender essential oil (antiseptic and smells nice)