That's strange. It could be Tuberculosis (I say this because you described the gut as being sucked in), and it is transmittable to humans so be very careful when touching anything in the tank just to be safe. I don't think all of your fish could have had this, but it is better to be safe than sorry. If it is, however, there is no cure known.
Hope this helps at least a little.
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Phishie
Site Coordinator

"An ounce of prevention is worth a pound of cure."