Metals dissolve very, very slowly into water, unless the water is acidic (for some metals). So, the only time you might get copper or lead in your water is when you have no water flow for a long time, then, without flushing out the piping first, you add tap water to your tank in large amounts. Water heaters use linings that protect the metal containers from the water. But, almost all houses use copper plumbing, most with lead based solder holding the connections together. So, in theory at least, if the water sits in that plumbing long enough it might dissolve some of those metals into the water. But, if you flush the plumbing for a few minutes before using the tap water in the tank, you should never have that problem.