I just can't recall how many floors down Atoll Reef is. But what I can recall was, the deeper the level went, the bigger the creatures appeared. It's just annoying that during our visit there, there were a lot of tourists. Some of the elderly tourists are either blocking the way or taking too much time in looking.
My advice is to turn off the flash and be patient with the creatures. Some moves very fast while other fishes are loving the camera.