Well, I'd pull it out of the pot and see if it needs more soil. I'm sure someone else will come in and help you also, but if you can put it outside (maybe on a stand) it will be much happier than in the house. You can always check it for bugs before taking it in for fall by shaking it and giving it a good spraying with the hose. As for the space at the bottom of the pot, it won't hurt the fern if you need to cut off the bottom of the fern if it has filled the pot. Take maybe 2" off, fill space with fresh soil, and replace fern. Fill in space around the fern with fresh soil.
More importantly, is it getting enough light?
