Standard unscented household bleach in a solution of 25 - 50 ppm (parts per million). No stronger or it may cause corrosion of the stainless. So that's 1 - 2 fluid oz (~25 - 50ml) in 19L or 5 US gallons. Fill it with water and add the bleach the night before you are going to fill it beer to give it an overnight soak, drain and give it a quick rinse with water just before filling it
