Just floss, I found the sponge to hard to rinse out. I never thought I was getting all the crap out of the middle. I have heard of people cutting the sponge in half and putting both halves in, easier to clean.
When dealing with filter floss just make sure that:
It doesn't have any chemical coating on it. If it says it's flame retardent then you don't want to use it.
Make sure you pull all the old floss weekly, a small clump got way down in the bottom of the chamber and I didn't realize it until my water parameters got out of whack. Now I use a smaller clump and make sure to get it all.
You didn't ask but:
Below the sponge I have Chemi-Pure, Middle Chamber I use Purigen (awesome, if you don't have it yet get it) & heater & Chamber 3 I upgraded the return pump to a maxijet 1200.
Are you just starting the Nano or have you had it up and running?