I personally fill my tank and use it for the week, then refill it with more THC eLiquid giving me another solid week of convenience. After that second week I will begin to notice small dust particles in the tank and the remaining solution is not as light in color (becoming dirty). This is due to the dust in your pockets, in the air, etc, making its way into the small holes your tank draws air in from. This in combination with the vaporizing coil within the clearomizer itself having a slight darkening effect on the solution it comes in contact with. Makes it necessary to clean your clearomizer tank every few weeks. After cleaning the tank once I’ll replace the clearomizer’s coil itself, basically making as good as new again. This improves taste and vapor production as expected.