I have two C3's running on my 38 gallon and and a C4 on my 29 gallon. Since 100 % of water flows through the media in the mechanical frame, the chemical chamber and biological tray need no futher "protection" with floss but the floss does make good "stuffing."
You will love operating your C3's in tandem. My water is so crystal clear & polished, it looks like I have a cannister filter and a UV
Sterilizer going just like this:
How to get crystal clear aquarium water - YouTube
But without the mess.
My recipe for flawless filtration with two C3's is as follows:
Cut to fit medka that works even better than the factory poly/foam pads. The C3 mechanical frame is just under 5 x 5 x 1/2" and I find the best results are obtained with the following cut to fit media:
Filter #1 running at full flow:
1) Acurel debris reducing pad (3/8 thick) +
2) AquaticLife 100 micron non-woven polishing pad (1/12th inch thick)
Filter #2 is set to run at lower flow and has a Fluval Edge Pre-Filter on the intake tube. Then in the mechanical frame:
1) Same Acurel debris reducing pad as filter #1 +
2) AquaticLife 50 micron non-woven polishing pad (1/12th inch thick)
[Note :100 micron pads are probably enough, but I recently started using the 50 micron variety in one of the two filters]
No good micron polishing pad can be rinsed clean and re-used.
The debris they collect is so fine it becomes embedded in the pad. Go here to this picture of my non-reusable OEM pad and read the caption ......
The factory pad is about 150 microns. The AquaticLife 100 micron pads clogged about every 3-4 weeks when I was running one C3 on my 29 gallon fully stocked tank. They cannot be re-used. Petland sells sheets of the 26" x 30" polishing pads for $10 so you are talking about 40 cents per filter per application (taking into account some waste in the cutting process). If you are running two filters, you probably will need to change it out only about every 6-7 weeks in a fully stocked tank.
Chemical Chamber: (both filters identical)
1) 100 ml bag of Seachem Purigen +
2) Seachem Matrix in a bio-bag topped off with floss.
NOTE: Purigen does not slow down the flow of water when used in the chemical chamber.
I use Factory C-Nodes and drip tray pursuant to the instructions. In one of the filters, I also added some seachem matrix to the trickle tray. Watch out for Matrix only in the bio-chamber. The Matrix "rocks" are bigger than the factory media and so you may not get as much bio-filtration when you try to fit them in a small space.
Here is a link to my photobucket account and the section called HOB
filtration that contains pictures of mechanical and biological media and chambers on my C4. Click on the relevant pictures and there is a description under each one.
[note: The unused mechanical media sitting on my desk in the picture is the recipe I use for my C3's.]
Also, it doesn't do any good to "overdo" the bio-media. You may want to watch this for an explanation and some additional information.