That was really gross. I wish you'd put up some kind of warning before I checked this out during my morning tea.
So, what size was it?
How many other were in the tank?
What size tank?
How was it behaving when you aquired it?
Did it behave any different before it died?
Was it eating well?
What were your water parameters?
These are just some questions I'd ask before I'd request an autopsy. I hope it was just a fluke, and the rest of your babies grow to be strong and healthy.