Yellow lab with brown spots

I have a E. Yellow Lab. that has developed brown spots on several
places. She doesn't show any signs of stress, but the spots persist.
She swims eats and bickers a little with the other fish. The water
tests 0 ppm on ammonia, nitrites, and nitrates remain very low with
regular water changes.

I haven't seen any similar spots on any other fish, so I'm wondering
what these might be or wear to look for leads.
She's been moved to a quarantine tank for now,but still shows no
other adverse affects that I can detect.

Thanks,
Mark