You leave it in the Tank unless you changing resin type and don’t have a spare tank, in which case you’d have to clean the tank of the existing resin. Which I suppose if you filter it through some cheesecloth or a screen, would be OK going back in to the cartridge through a funnel. The cartridges are said to be not-recommended-for-refill by Formlabs. But clearly, you can refill them by unscrewing the cap on top and pouring in more resin…