everyone who uses a clearo knows that topping juice can't go on forever since the juice becomes darker and darker and darker.. at one point (after the 5-th or sixth topping) one has to think what to do with the rests of dark e-liquid when refilling the tank with fresh juice. i personally just spill it in the sink when cleaning my tank. do you just waste the dark stuff? who knows how many carcinogens it contains.. so what are you doing when the mess gets really darkish?