I use Fluval (fish tank filter) and use a single cart for a week (Mon - Sun). I have maybe 15 or 20 carts currently in rotation.
Here's my routine:
I throw my used empty carts in a cup and, when I've collected enough of them, I soak them for a few hours (or overnight) in Listerine, rinse, and allow to dry. Then I restuff them all at once with Fluval.
Then on Monday morning, I take out the old Fluval from the cart used during the previous week, throw the cart in my cup, and grab another one I had previously restuffed. Then I fill it with liquid, and off I go.