maybe discus or angle tank..
Angle lol. It's crying out for some proper filteration and rays.
oh yes.. 4 HOB filters seem like a small thing for such a large tank.. looks like it is crying out for good lighting as well..
10 double 32 or 40 watt shop lights might do it justice.

and i would put a fish out of its misery, were it truly miserable. but you have to consider that this guy went out and bought a 350 gallon tank for this fish, instead of dumping it in a river somewhere. that's pretty admirable.
! You dont just kill a dog just because you cant find a home for it, now that would be crulty!