"Rust" type patches on my Giant Danios

[BalaKevin]

2 days ago one of my Giant Danios didn't seem right, and then I noticed a thin film in patches on it's skin. Yesterday it started to look like "rust". Today a second danio has quickly developed the same thing. So far the other 2 that I have are not showing any signs of it, but I'm afraid they will eventually. I also have 4 Bala sharks, 1 Pleco, 1 Bronze Cory. (they are all OK) 2 weeks ago I added 3 skunk cory's to my tank. I now have 1 left as the others suddenly perished over the last 2 weeks. I'm wondering if that might be the cause of my problem.
My tank is 50 gallons, non planted, with an Eheim Pro 2 filter. My water chemistry is perfect. I also run my Vortex Diatom almost daily for a few minutes. (moreso just to keep things "fresh" in the filter)
What would the sickness be?
And what should I do?
Am I running the Diatom filter too much?

I need help fast!

Thanks,
Kev-

 

[twoluvcats]

sounds like velvet to me...is the 'rust' kinda shiny?

Velvet is often brought on by too low of temps, do you have a heater in that tank?

Treatment: I've never had to treat it before, so i don't know what works best. Velvet is stubborn, and a cousin to ich, check out ich/anti-parasite meds and see if they treat for velvet too.

 

[BalaKevin]

Thanks.
In panic mode, I changed 15 gallons for now and turned up the heat. I'll be heading to my LFS and see what they can do for me. I don't want to lose my Balas! The danios can be replaced.