You don't really need the stress zyme (that is more or less the same as feeding them more food, just adds organics to the water) and you don't need to add chemicals to fight the ammonia (the water changes do a better job of this.)
What you do need to do is to continue the good job you're doing (I see you've now switched into high gear and realize it needs both testing and potentially water change activity sometimes twice a day.) You also may need to get past the feeling of stopping at 50% water changes. Many people get paranoid that a change of more than 50% is somehow too stressful to the fish but this is a misunderstanding. Ammonia or nitrite poisoning is much, much more stressful and the time when you can worry about the subtle effects of smaller percentage water changes are after everything is all well-cycled, not during a fish-in cycle and particularly not when stocked beyond about 25% of what full stocking (about an inch per US gallon) would be.
As said, you don't need to worry that bacteria are going out with water changes, they are multiplying very near to each other, within their biofilm structures which are like glue on the media surfaces inside the filter. Even amounts that our test kits measure as zero ppm are still enough ammonia to feed these bacteria and have them multiply (also the ammonia level within the filter will be higher than the level out within the tank.)
It will be best if the mature media you got from the store is positioned just before your new biomedia (your medium sponge for instance) in the water flow path inside your filter or if it is somehow interspersed with the new media (sometimes you have to be creative with scissors and sponges.)
KK's suggestions for re-homing/stocking changes still make a lot of sense for this situation.
Thanks for that. I really don't mind doing the water changes and it's also a steep learning curve. I just want the fish to be ok and be happy.
~~waterdrop~~
Welcome to TFF.