Keep lights off most of the time.. you can tell if he is stressed if his tank mates {if } any are bothering him} you can tell if he isn't swimming...but if he is cruising he is learning his bounderies of his new home.. he has to get use to everything.. and when he settles in his stress will go away.. you can still try to feed on and off until he finally eats.. but if there is no one else in the tank to eat it ..take out all uneaten food.. keep water up to par.. can you find out what he was eating before you got him??