If I were a betting man
I would put money on this tank possibly being skimmer less, with a canister filter.
I would also bet that there is not enough surface agitation and there Is a buildup of protein (looks like oily residue) on surface of aquarium. Lowering your oxygen levels stressing fish. Add a powerhead just under the surface of water spraying upwards and I bet in 30
Mins the fish look better.
Nitrates in a 2 month period won't be high enough.
Don't try dosing vodka until things are going good for a while and you learn more.
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