The filler probably won't last quite as long and you'll need to replace it when it starts hardening on top or doesn't fluff anymore. You can, and should, keep dripping onto it just to keep the atty wet. It's easy enough to replace and you can just use the same type of polyfil that's used for stuffed animals. It's cheap and readily available at most hobby/sewing stores. You can also use Fluval, which you can
buy at most pet stores. There are plenty of YouTube videos on different types of filler for carts.
Honestly, I gave up on carts and went straight to cartos. Much easier to maintain and cheaper to purchase. I keep the atties for dripping.