Because build-up residue can affect the flavor quality of your e-liquids, keeping your tank clean is essential. When e-juice has room to build up between refills, it covers your tank's coil, wicking material, and interior with an old, congealed layer of e-liquid. When refilling your tank with new juice, the old e-juice left on the coil and tank heats up and either completely overpowers or clouds the flavor of the new e-juice.