I use a coarse sponge like this:
https://www.aquariumcoop.com/products/sponge-pad-coarse
Cut that to fit where the filter cartridge goes. Then put some batting or filter floss on the output side of that. In the remaining space, some kind of biomedia. In mesh bags if possible.
I would agree with Al on everything else, but would suggest you only change lots of water until you get nitrate readings of about 20. Then let it set a week and see what's going on.
Regarding charcoal, I haven't used it for 20+ years.
I can't tell if those are live plants or not, but live plants can certainly help with keeping water parameters in check.
Also test your tap water to make sure you don't have a situation where you have some wild amount of nitrates in the water.