xtra filtr on a whisper

I'm a newbie here so cut me a little slack pleeese

i have a whisper power filter on my planted tank thats cycling now, and I know there are mixed feelings about these particulary inexpensive little filters, but thats what I got. My question is, since I won't be buying the activated carbon to go in the bio bag, what would be a good extra bit of biological filtration to put in there instead? Sponge? What else?

Thanks for any suggestions.