They may have suffered some trauma. I had two fantails that got stuck in
the plants and almost died. They showed those black marks for several
months, but the fish were fine. Of course the trauma they suffered likely
could have resulted in oxygen deprivation. I am just saying that it might
not be a matter of your water quality. They could have gotten stuck in a
plant pot or a rock or sucked up to the pump or something. good luck with
them.
"sean mckinney" wrote in message
...

I recollect black marks developing being a sympyom of an ammoni-x burn
you might want to check the NO2, NO3, NH3 and NH4 readings. Surf those
and take appropriate action




--
sean mckinney