How do you bathe a fish?
That's like asking what one should add to powdered water.
Seriously, though, the platies look beautiful in the pics - I can't see anything amiss at all. Maybe what you're seeing is just a few scuffed scales?
I can't imaging algae being able to grow on a fish on account of the slime coat.
, I think you're just being an worried fish-mom
. I've never seen any mickey mouse platies at my lfs
- he's so colorful!