I agree with lack of oxygen. All signs point to it. No filtration(lack of water movement), fish circling or laying down, the picture of the tang's mouth open. Everything points to it. My only other idea beyond yours, would be how the water is made. Is it RODI? Do you use SAFE or Prime to neutralize chlorine? I have lost my tank to lack of oxygen and everything he is saying and in the pictures and video all are nearly identical to my disaster :/These fish are becoming Moribund and are rapidly declining. Was this tub rinsed/cleaned prior to use?
Other than airstone, i see no means of biological or mechanical filtration and am convinced on water being the issue-low oxygen- likely elevated ammonia- lack of water movement and surface air exchange and foaming at surface is noticeable
It is super upsetting and saddening.
@NemoEnthusiast please dont let this discourage you. Its these hurtful, but memorable disasters that make us better in the hobby