The only way in freshwater to have 0 nitrate would be either very heavy planted / lightly stocked with fish OR uncycled tank. Uncycled
My guess is that you are not performing the nitrate test correctly. You have to follow the directions exactly. No way that stock isn't producing nitrates
I saw some for sale today that were about 2 inch or 3 inch, so I would not be surprised that that one is that big.
I compared them today and both looked the same color to me.