yeah, if you can get a recent shot (not video still) that would help. if the fish looks almost black there is a good chance something is wrong, but it's hard to advise you about it accurately without a pic.
new filtration should be good. what are powdered bioballs? what are they powdered with? either way, i would avoid adding new fish in (as apparently the arow was added since the last disaster) until things are sorted out, that includes temporarily housing some other big fish.
is the Cuban eating? are its eyes cloudy? what are your parameters right now? i've never seen a large cuban go really dark without some sort of stress factor, even in shots of other large Cubans. good luck--
--solomon