Unfortunately, without a necropsy, there is no surefire culprit for the
symptoms you describe.
Hemorrhage, ulcers, and bloody streaks are common symptoms of a many
fish pathogens including infections by the aeromonads and mycobacteria
(etc.) that are generally prevalent in zebrafish systems.
My inclination would be to investigate the environmental (water
quality, stocking density, frequency of usage) and nutritional stresses
on the fish that might be compromising their immune systems.
I would also be sure not to return the fish in quarantine back to your
systems should they every appear healthy again. If euthanasia is not
an option, at least consider them subclinically infected with possibly
virulent bacteria for the rest of their time in your facility.
Fish Facility Technician
Brigham and Women's Hospital