I suppose it would be fairest to contact a manufacturer and ask them.

In the meantime, the gravel, "slightly driven" filter floss or water from an established, healthy tank might be easier to come by. Somewhere in this world I have about 7 three gallon carboys. One is at home. The others were filled with starter water or R.O. water which I sent home with friends. In time I know they will be returned - maybe when more water is needed. ;)

That does me a service too in that by giving away water from that tank, I have to change in some more clean water. :)

It is almost impossible to do a partial water change too many days.

The activated charcoal is good for a few days to two weeks as an "Adsorber" of ammonia. As surface area gets taken up, it become ineffective for that purpose. The charcoal could help by providing surface as a biological filter or as drainage at the bottom of a flowerpot.

I'm a bad influence in that I keep suggesting ways for people to spend more of their time and money. (Ya shoulda see me at this auction yesterday.)

But a 10 gallon (really a little over 8.5 gallons or a little over 30 liter) tank or (even better) a "20" gallon tank are better starter tanks. They are much more stable and less likely to get polluted, chilled, too acidic, or otherwise out of control compared to a 2.5.

The smaller the container, the easier it is to kill fish.

I still have about a dozen 2.5s in service, but my fish are much healthier in larger tanks. They are less aggressive in larger tanks. Their movement in larger tanks verges upon the poetic. Plants grow better in larger tanks. And many more fry survive and grow proportionately larger, more quickly in those larger tanks.