i used to have a 3gal pico and i loved it, i bought a Marineland Penguin 100 to replace the rinky dink filter it came with and it kicked butt imo, not too expensive, i know some people say that the bio wheel is not good but i had no problems with mine just my 2 cents, good luck!
Thanks for the replys. My question is more towards the media to use. The 6 gal comes with a sponge in the overflow section. I was thinking of replacing it with something different.