I picked up a CFS from eBay. Great filter. Very quiet, and has a wonderful flow rate. The only media inside are sponges. What should I go with to get some biological filtration along with it?
Sponges will provide biological filtration also.
I have the CFS 1000. It came only half filled with sponges. I bought another replacement sponge kit for it, and added those and a bunch of filter wool to fill it up. It's been running a few weeks now (it's running on my current 6ft tank so it will be ready to go when my new 6ft tank is set up) and in not entirely sure the snug sponges are efficient. The pump, being on the bottom, pulls the water from the top down. So you'd assume that as the top sponge becomes heavily clogged the sponges below it are not going to be fully utilized.
My top layer, which is filter wool, is REALLY dirty, and I'm going to throw it out. The sponge below it is pretty dirty, and needs a rinse, But the rest of the sponges appear to have very little dirt in them. I think this may be because the filter isn't utilizing the entire surface area of the sponges. I won't know until I pull it apart, but I think only the centre of the sponges is being used because of how the sponges are designed to fit snugly to the filter barrel.
I will probably take a knife to the sponges and make some modifications to improve flow around the sponges, and throw a couple of bags of matrix on the bottom by the pump.
I'd be interested to hear your experiences with the CFS.
it's not staying where it is in the photo, I'm just maturing it ready for when this other 6ft tank get up and running! 
