how long has the tank been running for,,you need to keep doing daily water changes until you get this under control. I would go 50% wc and add prime. When was the last time you checked your filter. Running only 1 filter is not good imo.
get rid of the carbon,and add more bio rings,,biomax or whatever your lfs carries. i would also consider a second filter a hob such as ac110 or even a large sponge filter. will help a lot with your bio-filtration.
Yeah i know kaiser dude the op tank has probably never finished cycling, or it has crashed for some reason. But he has got quite a few fish in not a very large tank. Adding a extra filter is a safety net and running multiple filters on a tank is imo always a good idea.I like your avatar, but question your advice.
Not sure how more filtration, let alone media will do anything for nitrate.
This tank needs a WC schedule in keeping with the fish load.
It needs an owner/operator who has a better handle on the nitrogen cycle and how it works in his tank.
While getting up to speed on the chemical happenings in a fish tank, I would recommend WC as a way to keep those fish alive and healthy.
Feed lightly and WC your way to a healthy home for your fish.
Good Luck.