weird black spots

[Jht]

on my 2 loaches they have developed faint grey/black patches along their backs i have had them for about 2 weeks now they are eating fine and are a lot more active the black marks have been there for about 1 1/2 weeks now

 

[shimanocono]

Can you explain the black marks better? I just noticed my black tetra has some black on his tail a bit it sort of looks like ink...