I am really sorry about this. I had the same problem before, I had some very thin and very weak new neon from them. I did point this out to the sales immediately, but he just said it is not a problem at all. The fish, of course, died soon after I put it in my tank.
I just don't know why I can't choose the fish which I pay for
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There is another problem in this shop, it is very dark there. Even you have a chance to check the fish which is picked up by the sales, it is impossible to check whether there is any defect in the fish in this bloody dark environment. Once you walk out from the shop and discover the defect, sorry! the fish is yours now. What you can do is return to home with the poor fish in a bad mood. Now, thanks to the helpful sales in Fat X, my tank has a abnormally thin and deformed checkbroad. Fortunately, it still swimming happily all the time and this is only thing that can cheer me up in this occasion.