Hi all. TLDR at the bottom.

So to start it's a 55 gallon and here's what's in my tank: 2 Angel fish, about 16 small tetras, 5 platinum hatchet fish, 9 zebra danios, 3 otos, 1 albino pleco, 9 assorted corys, and a bunch of mystery snails.

It's a planted tank also with quite a few plants.

I thought I had a Marineland 360 on it but comparing physical measurements of the filter (there's absolutely no indication that I could find on it that told you the model number) I realized it was a 220. Purchased a 360 and installed it Thursday, transferring the media from the 220.

Aside from sky high nitrates the nitrites read zero, the PH is a little on the low side (just under 7) and I'm slowly trying to raise that with alkaline buffer though all the water changes aren't helping. There's a small amount of ammonia, but it's just above zero (forget what the ppm on the chart are, but it's what ever is just above zero).

I have to admit I was a bad aquarium owner as a hadn't checked my nitrates in a long while before I made some changes. Everything had been so stable for literally years and I was just topping off water.

The things I did were adding black blasting sand as a substrate on top of the original play sand, adding flourish root tabs, replanting the tank, and adding stock. I basically doubled the number of tetras, added 5 corys, 4 danios, 2 otos, and 4 hatchet fish.

The fish all 'seem' fine. They're active, vibrant in color and not hanging out in weird places or dieing. I feed them once a day and only about what they can eat in a minute. Anything that sinks gets taken care of by the corys, snails and pleco.

The nitrates seemed to be reading upward of 160ppm as near as I could tell based on the color. Possibly 80, it's not 100% clear to me.

My tap water reads 0 nitrates.

I was doing 50% water changes three times a week with little effect (I wasnt checking the water immediately after the water changes but instead before the next water change). I eventually bought a nitrate absorbing pouch to add to my filter. After two days I tested the tank water and it was reading zero to nearly zero nitrates. I took it out to recharge it after 5 days and left it out for a couple and the nitrates sky rocketed again. I didn't put it back in because I was told it's not a 'natural way' to deal with the nitrates and is bad long term.

I did a 50% water change yesterday and tested the tank immediately after. I seemed to read around 80ppm maybe 40ppm, which I guess would make sense if it was 80ppm - 160ppm before.

I did another 50% water change today and it seems to be reading around 40ppm. Which, again, I guess might make sense.

So is it as simple as my tank is overstocked? Am I actually over feeding? Do flourish root tabs increase nitrates for weeks? Can't see how the substrate would affect nitrates.

I ordered some bacteria starting product thinking maybe the tank is lacking something on that end that needs boosting.

TLDR; 55 gallon planted aquarium with a Marine Land 360 filter stocked with 2 angel fish, 16 small tetras, 9 zebra danios, 5 platinum Hatchet fish, 3 otos, 1 albino pleco, 9 assorted corys, and a bunch of snails seems to be consistently reading 80-160ppm nitrates. All other parameters are basically fine. Did add a bunch of flourish root tabs but that was a month ago. Water changes have minimal impact, only thing that made a big difference was a nitrate absorbing pouch added to the filter. Feeding once a day, basically what they can eat in a minute.