ro/di canister?

so i have finally gotten my ro/di system it is the 5 stage from bulk reef supply with dual di canisters. i have been looking around and have a question that i have not been able to find the answer for. when the system is not in use do i just leave the water sitting in the cansiters or do i screw them off and drain them after every time i use them. i am asuming that you dont since i havent found the answer to this question but i just want to make sure so that i dont mess up something and end up having to buy new filters for the system already...on another note i will hopefully also be getting my 150 gallon tank this weekend and getting to work on the stand and canopy for it...just an fyi

 

on most r/o units they have a waste water outlet and you shouldnt have to empty the canisters.