Black streaks

[b0ssgirl23]

Hi there, can anyone offer me advice on my pet goldfish? As of a few days ago I noticed my goldfish (garfield) has developed black streaks on his fins? Hes currently in a 165litre tank with a few other goldfish my water quality is perfect , but his behaviour and appetite has changed and I can't figure out why please help!! Thanks in advance

 

[Delapool]

What are readings? That would be first one to check for say chemical burns eg ammonia which can take several days to show.