You can see the proof in the image (click to enlarge) and read the great news it says: “You’ve discovered a new level of achievement! Earn the Diamond award to show your friends that you’re a FishVille master and earn a special reward!”
I am not sure at the moment if all the achievement have this Diamond award level, but I am tempted to believe that they do: if you take a look at the shells representing the achievement levels, you’ll notice that there’s still a spot for one more near each achievement. And add to that the fact that we’re going to receive a “special reward” for getting the Diamond achievement in Fishville, and I’m ready to forgive Zynga for the poor Fishville presents!
But I still wonder what could the special Diamond achievement award be…