Ok so I have good news and some moderate bad news for you.
The good news is that your filter has a strong and thriving culture of bacteria. The moderate bad news here is "Beefheart." Don't get me wrong, you shouldn't stop feeding it to your discus but at the same time you have to realize that Beefheart (and the other meats a beefheart blend contains) are more protein rich and in short, result in more nitrates at the end of the nitrogen cycle.
I'm using both a fish mix and beefheart mix and going from a planted tank, that was fed flake food, to beefheart was a "shock" in terms of the levels of nitrates it created. My advice? Two water changes a week and employ some faster growing, nitrate consuming plants. If you chose to add a fertilizer to boost growth use one that doesn't have macros like nitrate.
Lastly, and I say this with some trepidation considering your tank is beautiful, I'd remove the large flat/grey stones. At a glance, they look like they're creating a lot of crevices which could trap uneaten food and also be a breeding ground for bacteria and dead spots. That said, I am no expert. So please feel free to disregard any advice that doesn't suit your vision for your aquarium.