How long has the tank been cycled?
Edit: after reading some of your other threads, IMO
I don't think its fungal
When you had a problem with your nitrates before it can be caused by ammonia breaking down into nitrates. Causing ammonia burns on its wen. On an oranda this is where ammonia burns show up.
The dark marks on the wen can show up after a growth spurt and they can take some time to fade away.