Full grown oscar died... why?

I bought this oscar at its current size from my lfs back at the end of july. It didn't eat for me for a couple weeks but after that was perfect.
Background: 165gallon tank with oscar, jd, and silver dollars. Tank is around 78 degrees, filtered by 2 fluval fx6, gets weekly water change.
It ate hikari massivore sinking pellets daily and was very active for the past month +

Two days ago he stopped eating but he ate alot the day before that so I wasnt concerned. At that same time he stopped eating he started hanging out at the top of the tank just opening and closing its mouth but it wasnt until today that behavior actually concerned me, I saw the silvers also breathing rapidly so I thought it might be low oxygen. I did a 50% wc (today) and added an aerator to the tank. The silvers improved but he stayed at the top mostly, but started swimming around more. Saw him alive at 930pm tonight at the top and found him dead at 11pm. He was laying on the ground (upright) but not moving, concerned I put a net in to touch him to get him to move and it just flopped over on its side. Grabbed it with my hand pulled it out of the water and examined it (im in vet school so I dissected it to check for any grossly visible abnormalities). Zero outward signs of disease (see pic) and nothing grossly visible on the inside (no major parasites etc). He was very thick and seemingly healthy. No idea how old he was . Any explanations you guys can come up with ?


The only thing additional to note is over the past few days it seemed like he had some excessive slime coat. Idk guys its baffling me.

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Heres the result of my water test: everything appears normal except the nitrate red could be anything from 40-80 I cant distinguish between the two shades of red honestly heres a pic.
So it wasnt (or doesnt appear to be) ammonia or nitrite. The ph is approximately 7.5. All other fish appear normal this morning.

Was not getting picked on by the jack I watched them extensively as when I put them in together in july they were both adults so I was paranoid about them fighting. They actually got along really well for both having a rep for being aggressive. Additionally he had no battle wounds ever

Any thoughts?

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I Feed the tank algae wafers but he spit those out but good to know u was under the impression massivore was great for them.

Also would a change like that occur so rapidly in a fish that size? He only ate consistently for a month or so

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Is massivore suitable for my jack dempsy or should I also provide him an alternative food? He also wont touch the algae wafers.
What was the gasping for at the end of life? It seems to becoming more and more clear that his death was not a result of the tank itself but something about him. Upon dissection his liver did not appear fatty (by mammal standards i.e. it was not yellow/ rubbery rather it was green). While I do not know how it works with fish generally the dietary problems take quite a while to manifest and he was not overweight.

I will be buying another oscar whenever I find one that I like and this time ill be sure to not feed massivore exclusively but will get something along the lines of hikari gold food, is this better?