Cleaner shrimp spots

[Leedowlrods]

Hi, my new cleaner shrimps (3 weeks) have recently molted and today they've developed black spots over their body, I did a 50 % water change yesterday due to high nitrates. My RO water had too much SG so I diluted it with tap water,
2 l Tap water mixed into 6l RO water
Is this the reason for the black spots?

 

[Crystal A]

I saw this in another thread. Google taura syndrome virus. Someone else's shrimp had the same thing and this was all they could find to explain it. The thread is under the sick fish category. Shrimp with black spots I think it's called.