Many use a citric acid solution, but it doesn't get the brass really shiny like tumbling. A product called Lemi-Shine is sold in supermarkets for use in dishwashers, and it is nothing but citric acid with an anti-caking agent added. About a heaping tablespoon in a gallon of water will do it. Some feel a squirt of liquid dishwashing detergent like Dawn helps the solution. Soaking time is not critical. I usually let cases soak overnight.