Sorry to hear you lost a fish....its odd how we can get attached to them so quickly.
My guess is that the neon had perished PRIOR to getting sucked up into the intake tube. Neons LOVE a nice strong current so I can't imagine that it wasn't able to swim against th current from the filter.
My guess is that you just got an unhealthy fish. Just remember, Neons are super sensative to water conditions and really like well established aquariums.
But imho, having 9 is better than 10. I can't remember where I read it, but I recall seeing that neons liked to be in odd numbers.
(I keep 7 in one of my 10s and 5 in my 55)
EDIT: To reduce
ammonia levels, I suggest partial water changes (like 20%) with Prime for about a week or so. That should remove your ammonia levels