I was wondering if anybody knows or has used another brand filter cartridge in the penguin 350
You can use the Aqua-Tech brand #3 filter media. Available at your local Wal-Mart.
It's exactly the same as Marineland's Rite-Size "C".
Also, Aqua-Tech #1 = Rite-Size "A" and
Aqua-Tech #2 = Rite-Size "B".