Interesting question as to whether bacteria live in carbon, I'd say if it does it would be a small colony. Hopefully a resident expert will weight in.
A sponge and biomedia are what you really want to seed it properly, but now that we know you have a seeded 2nd filter, you're in good shape. That filter will help seed the new one quicker. If you have filter floss, or something along those lines already, you could add that to your AC50. It doesn't "have" to be their media (sponge/biomax). That's the versitility of the AC basket design.
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