Tang flipping out?

[tropicana]

I have a sailfin tang that is going crazy darting in and out of the rock work he was fine then all of a sudden snap cutting and weaving about im afraid if i cut the light hes gonna crash into a rock and die. Its a FOWLR have two other tangs a Yellow and a Scopas, Foxface two ocellaris clowns. Only issues would be my nitrates are high.

Thanks
Frank

Fish can see pretty good in the dark too, But it calms them down, he wont dart like that in the dark though.