Nope. I would feel bad if I did that to a hobbyist, we'll I mean I wouldn't do it, but I don't feel bad for you doing it to petco. A) have you seen the recent thread about where they get their fish from? B) you know the markup they make on fish? C) it's their responsibility to label and sell their products appropriately, including training their employees, d) they make enough money off people, e) it takes two parties to complete a transaction, you didn't steal this. I would feel really good about myself, don't lose an ounce of sleep over the petco ceo, he is not missing the difference.
My answer would be totally different if you did this to a hobbyist though, taking advantage of an individually is different than taking advantage of a large company.