Black Spots/colouring Identification?

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Hi,
I fairly recently added four mollys to my tank, I noticed on one's tail he had a small black patch/spec - it was so vivid it looked like colouring so assumed it was just a pigment disorder in his skin, I've since noticed he now has a similar one on his other side and a few minute specs of black coming up.  Other fish also seem to have very slight black outlines around their outer fins.
 
I've tested the PH and ammonia today and both good, as I read it could be 'ammonia burns'.
 
I'll try and up a picture when I work out my new phone.
 
The fish are all behaving normally.  Except perhaps the molly described above, he seems to be swimming round biting floating debris then immediately spitting it out (continuously).
 
Any help much appreciated, I'm fairly new to fish so don't want to underestimate things like this/not act quick enough.

 

[TropicalDrop]

Here we go.
Molly with black mark on tail and black spec on body:

 
dark fins on betta fish:

 
Dark fin and tail section on red fin shark:

 

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If anyone can comment it'd be appreciated, little black spots appeared on other mollys today