I only have my 10 gallon up and running at the moment but I use an AquaClear 50 on it, which is the best hang-on filter btw. Anyhow, I use a bag of Seachem Purigen, a bag of Seachem Matrix, and one foam pad. I highly recommend using Purigen and Matrix, they are both great products. Filter floss is good for biological filtration, but not for mechanical. It will definitely need to be changed often if you do not have something else to catch the particles and waste.
Use a nice sized foam pad as your first filter media (the first part the water hits when it goes into the canister, then use a bag of matrix or similar high surface area medium for bacteria to grow on, then use some purigen to keep your water crystal clear.
Make sure you do not change your entire medium at once as this can cause a spike in ammonia because you are removing a lot of bacteria. Change one medium at a time...
Good luck,
Ryan