Sick Cardinal Tetras

[willowmidnight]

Left my sister in charge of the tank for a month while I was away, and while the tank water parameters seemed quite good she hadn't vacuumed the gravel very well

Ammonia:0
Nitrite:0
Nitrate:20

I decided to do a thorough vacuuming and examine my fish

All seem well, except for two cardinal tetras
They look like someone has taken dark blue/black paint and dabbed it over random parts of their bodies. not furry, so I don't think it is fungus.

Occasionally, the other cardinals chase them

Any ideas people?
Thanks in advance

 

[WILDER]

Black patches can be ammonia burns that are healing.

Any signs of flicking and rubbing. darting?
Just make sure it dosn't look like a grey bluish film.

 

[willowmidnight]

Ok I will keep an eye on it and see if it gets better or worse. Thanks Wilder

 

[willowmidnight]

It kind of does look like a blue film though
No flicking or rubbing though

 

[willowmidnight]

Getting worse. The patches seem to be getting larger and thicker.
Noticed at night time when cardinal coloration fades the dark blue remains.
3 of my Cardinals have large patches, 4 have small patches and the rest (4 others) seem to be fine for now.
Doesn't look like it is on any of my ornate tetras or my Bristlenose catfish.

I don't know what to treat it with. So far just keeping up with regular water changes

 

[willowmidnight]

Has spread to a few of my ornate tetras. Starting to wonder if it might be velvet as they look like they have a dusting of small pale spots as well as small blue patches. 2 of the sick cardinals now have grey patches as well. Gonna have to find some medication when my pay comes in tomorrow

Does this sound like velvet? Or would I be better off trying a broad spectrum medication?