You can refill the same cartridge, just make sure you clean out both the inside of the cartridge and the filler material. However, the stock filler is only good for a few refills and doesn't work very good anyway, so I'd suggest you go down to your local Petco or Petsmart or any pet supply store and pick up some Marineland Blue Foam Aquarium Filters size U sleeves. You get 3 sleeves for around $10, which is probably enough foam to refill well over 1000 cartridges.
You don't need to fill your cart with blue foam, just cut a piece about 1.5x the size of the hole in the cartridge and pop it in the top, it acts like a plug to keep the
juice from spilling out, but wicks much better than that garbage stock filler. Make sure you don't use too big of a piece or it won't wick properly, but too small of a piece will let too much
juice through and you'll flood the atty. You'll get used to how big of a piece you need for your plug, so don't worry about it being a hassle, as it's much easier than dealing with stock filler.
Using blue foam, you can
vape every last drop out of the cartridge, and you'll know when you need to refill by the taste. Quality-wise, this stuff is leaps and bounds above that stock filler, as it works better in every regard.
