I'm gonna play a little devil's advocate here, because I think what you did here was wrong. Since you knew better, you should have told the guy which ones were the males and the females and how to tell them apart. First and foremost, this is the kind of knowledge a new employee needs. You could and should have done this guy a great service by teaching him what proper fishkeeping is. You could have told him how important keeping the correct sex ratios are. You could have told him so that next time when he sells fish to people, he could them tell them, and then those customers can keep their fish healthier and happier. Your knowledge could have gone a long way towards helping a lot of his future customers keep healthier fish.
But, secondly, because you knew better, you basically have stolen goods from a store. You knew what the fish really were, you knew what the prices were supposed to be. You may not have done this physically, but basically what you did here was take the price tag off of a cheaper item and place it over the price tag of a costlier item, so that you get the costlier item at a cheaper price. In a word, this is stealing. People make mistakes, and if you didn't know any better, and then got home and found this out, I wouldn't have a problem with it. But, from your story, you knew exactly what was happening, and I think it was dishonest of you to pay the wrong price. If you couldn't afford the fish you wanted, either don't buy them or buy something else. Preying on the ignorance of an employee is the same as stealing.
Finally, Petsmart isn't just going to take this loss, small though it is. They will just raise prices on their other fish or products to make up for this loss. That loss will be accounted for somewhere else. So, everyone, next time you think to yourself "why are fish and equipment so expensive?", you just have to remember this little story by krib. You should have paid what was a fair price.
I don't think that this is something to be proud of krib. I think that it is something to be ashamed of. I hope you don't think of this as a lecture, because that is not how I intend this to sound; but I'd hope that this has inspired you to look at this from a different point of view than just your own personal gain. If you had found a good fish that was on sale, or Petsmart had it listed at a price cheaper than their competitors, that would be a good find, in my book. But, like I said above, in my opinion, because you knew exactly what the employee was doing and didn't tell him, it amounts to theft.
I am very interested to read what other people think.