I disagree completely. Many fish stores have knowledgeable employees who you certainly should listen to, just do not limit your self to their opinions alone, do research on your own as well. And as far as there being something wrong with a fish if it doesn't eat for two days that is flat out wrong as well. While most clowns are vigorous eaters, juveniles tend not to be as voracious, especially in a newly established tank, by themselves. I would not panic, simply keep a close eye on the fish, and try a few different kinds of food, give him up to a week, most people drastically overfeed their fish, most of which only need to be fed once a week.
oh and the nitrate spike is from the added bioload, and all that crap that your fish keeps making.