Generally if possible, use it as fresh as you can, if not, a bag is fine.
There's not much food for the bacteria, but it'll bounch back fast, just like the sealed stuff that they sell like Bacteria Cycle etc, but it'll do better because unlike the Cycle, it has the specific bacterial decomposers that break down plant waste/detritus, not just fish waste/NH4.

Regards,
Tom Barr