I use about a half cup in the laundry sink... that might hold 20 gal of water and a bucket full of rocks. I use a 1" piece of PVC pipe I drilled some drain holes in (most of the way up the pipe) so the water can drain without plugging the sink. If I want to hold the water in, I have a 1 1/2" solid piece of Pvc pipe that slides over the drilled one and it keeps the water in. Then when I want to drain the water, I just slide the larger pipe up and out and the smaller one actually drains. I generally rinse twice and I'm done.

kL