Nope extra filter needed, I only use sponge filters in my present tank and no chemical filtration. I'm only filtering with peat and adding botanicals, if you want pure blackwater, then you have to add it to maintain the colour in a aquarium.
Rainwater is another great resource you can use as Stephen mentioned, I don't use it purely now because we live right next to the sea.
Vale can tell you more, @Vale! ...you have to shout @Stephen, think he's hard of hearing these days.