My axolotl died. Why?

[Cocoapanda10]

I got 24 axolotl spawn on 24th May , it was laid on the 17th May 2012, and on 28th May 2012 4 of them hatched. Then they started hatching like crazy yesterday, with 6 new hatchlings alive and well. Then, just before I went to bed, I was watching them, and one Axolotl started moving, swishing his tail and wiggling his head, and then emerged out of the spawn.
But he didn't swim away like the others did. He just sank onto his side.

I poked the ground beside him and got him onto his correct position, and he swam away, not as fast as the others, and he seemed to be shaking. He landed onto the substrate, correct position and all, and I noticed his tail was arched, and he had white on his neck.

I tried to see if he had made it through the night today, and he had. I went to school, and came back. I went to the dentist, and finally stayed at home. I rushed into my room to check him.
Unfortunately he was curled up into a ball, in a different place as to when I left him. My mum poked the ground beside him with the small fishing net I used. He didn't move at all. Didn't even uncurl his tail. It was pretty obvious he was dead.

I have a deformed hatchling right now, and he completes a set of 19 axolotls (2 are hiding) and 5 spawn have not hatched yet but the embryos are doing fine. Why did this one axolotl die?

I checked the temperature, the ammonia, the nitrate, the everything. And his siblings are doing fine.


EDIT: All of the eggs have hatched under the last 5 minutes.

[nai live]

Not all hatchlings are viable, out of my last batch of 400, thirty died for no apparent reason. The fatalities generally happen within the first couple of weeks, after that the surviving ones should be ok as long as they are cared for properly.