I don't. However, I took at least one LARGE handful of BioMax from my FX5 that is running on a heavily stocked 100 gallon, and put that in the filter of the 35 gallon, in addition to a sponge that was used in the tank when I just had a few fish in there(so the biomax can seed the sponge). That should have been more than enough for the 30 or so Neons in there. Plus I have done waterchanges every other day. Doesn't matter if it is a large water change or a small one, I still have fish die and the Molly still acts very stressed. There's plenty of hiding places with at least 5 fake plants and 1 large driftwood branch on the bottom of the tank, plus a floating piece of driftwood and 2 floating plants. I can't figure out what I'm doing wrong...I only ever have problems with fish in this tank(QT tank), never in my main tank.
if you're depending on the biomax to seed a sponge, that means you already think more BB is needed in that QT tank.
That is not the method or point of using sponge filters. the way to do it is you have a sponge going in mature tank for long time so it is already also mature, then put that in the QT.
Also, doing water changes every
other day tells me you intuitively know that the tank is still cycling & you start out with an approach to hopefully do damage control before fish are smacked by stress.
It isn't working & this is not your first rodeo, no offense intended.
Go in your fx5 and take out more BB. situate that at the beginning side of your new filter media so water pushes it through there.
That sponge filter is useless/zero/nada quotient for quite a while.
Medicating, even if necessary, will additionally stress the tetras. Best to first guarantee the BB aspect of tank. Your fx5 won't miss it at all.
When I take media out of a mature filter for another tank, I add the same amount of new media back in there.