Aquariums: What to change out/replace?

Hey folks, my son and I started a 20 gallon freshwater tank with live plants about a month and a half ago. We started with a fishless cycle and over the last month have added 9 residents. All this to say we have had a really healthy tank, our only casualty was a tetra who died in transit home.

Coming up on the one month mark since adding fish (and one hitchhiker snail) I wanted feedback on which parts to replace. I’ve looked at the manual for our filter and can’t seem to really pin down where all the “good bacteria” live. I believe they are in the front sponge of this aqueon filter, and I’m aware that we shouldn’t change which ever media is the designated bacteria home.

That being said, the filter cartridge in the back is a carbon filter. I’ve read that when these filters hit a certain age, the carbon is gone and the bad stuff it filter starts leeching into the water. I’ve also read a lot of the stuff the manufacturer/seller says is hooy to sell more products.

I’m just looking for some real hobbyist feedback on which of these parts I should be look to replace, because maintaining our healthy fish tank is obviously paramount to us.

For the record, disregard the water color, lol. We messed up the drift wood boiling a little bit, and we have some good ol tannin water now. But all test levels read normal and I’ve read the tannin water isn’t a bad thing. We’re hoping the water gets clearer over time but not going to lose any sleep over it.