Mr. Fish is really just a toy to the owner which is why the stock is so light. I am not saying that he does not care about his store, as you have seen it is on a corporate worthy size scale and VERY clean. Its point though is not so much to make money as just to be able to make some people happy and generate enough flow to keep it open.
I have never seen much rare anything there because of it, but sometimes they do and also if you shop long enough may talk them into ordering if available. I used to buy most of my animals from there when I lived in Mandeville so I recommend it, I actually replicated their pond on a slightly smaller scale at my parents because I was so impressed with it.