Help with scales issue

[F4A]

I've noticed a few of my platy have badly damaged scales, zebra danio also looking in a similar condition. I've attached a photo. Please help me ID the issue.

Also, I've noticed a lot of slime building up in my canister tubes over the last week. I'm not sure if this is connected but its causing a right mess. Is this normal?

 

[BettaGal]

At first look, it looks like that one is injured, although the picture quality is not great. Post your water parameter, all tank mates, and if your tank is planted. Any strange behaviors you've noticed lately.