Tanks collect condensation, this is normal, and can cause e-juice to build up and stick to the walls of the tank or even clog up the tip.
There’s a simple and quick way to clean your tank. Once it’s empty run it under warm water, make sure to close your drain so you don’t loose your cap, for a few minutes. Then blow through the tip a few times to make sure it’s clean. Put it aside and let dry. That’s all!
Tip courtesy of @DoctorVapor