Black marks on my moorish idol and powder blue

[Lefeta]

Hey everyone, just wanted to know whether anyone has experienced this before, and if its normal? I’m just concerned because my powder blue had similar marks 2-3 weeks before dying of an unknown cause. I’ve had the Moorish Idol for almost a week now, but his been eating well and has showed no signs of weakness or irregularities.
Tank:
- 350L
- Salinity: 1.025
- Temperature: 25 Degrees celsius
- Ammonia = 0
- Nitrates = 8ppm
- Calcium = 400ppm

 

[F i s h y]

Not sure what it is but you might have better luck in the disease forum.
Free bump. Hopefully someone else can help.

 

[Lefeta]

Not sure what it is but you might have better luck in the disease forum.
Free bump. Hopefully someone else can help.

Hey, thanks for responding, still fairly new to this thread. I’ll create another post under that forum. Thanks!