any soap that is actually soap (fat and sodium hydroxide) . If hands are washed and just as importantly rinsed properly there is little chance of anything being left from soap (that was soap does suspend and encapsulate dirt, oil, bacteria ect in water and washes it away hydrophilically with rinsing, hence the dirt leaves and stays out of your clothes and goes with the water in a washing machine). this is why "anti bacterial" soap is one of those useless inventions that serves marketing more than function (calcium supplemented anti-acids fall into that category as well since calcium isn't efficiently absorbed once you neutralize stomach acids. a few things I learned in nursing school). washed properly 99% of bacteria is already removed and the antibacterial agents arent in contact with your skin long enough to do anything anyway unless you wash your hands for several minutes. If regular soap wasn't efficient in removing bacteria anti-bacterial soap would have been a mainstay a long time ago by neccessity. anyhow what ever is left on your hands is either from hard water, improper rinsing, chemically unsound soap or attached to your hands strong enough to not come off in your tank. soap scum ect is from hard rinse water and/or incomplete rinsing.
