flwr, some carbon filters are made in such a way that you can replace the carbon inside when it loses effectiveness. The one you have (and most of the low-cost models) are meant to be disposable. It's possible that it could be opened up and refilled, but it would take some work. Make sure you're not pushing air through it too fast and that you've got all the connections taped up well. I had a tiny gap where the duct met the filter and it was enough to let out quite a bit of odor. Otherwise, sticking some ONA gel in the area should be fine if the filter is handling most of the smell.
The plants are looking great!