Black fungus?

[EliK]

This morning, I noticed these spots on two of my tetras:

(Three pictures of the same fish) - red glofish skirt tetra, a little less than 2 inches long, more than 2 years old

Black skirt tetra, about 1.5 inches, about 1 and a half years old. I'm not sure that this is actually the same thing as on the red one, since this spot does not seem raised as much.

Tank parameters:
82.5'F
0 ammonia
0 nitrite
<20 nitrate
Stock: 3 skirt tetras (2 glofish, 1 natural), 1 5" pleco, 1 angelfish
I do a 50% water change every 7-10 days.

I had a couple more tetras in the tank (natural and albino), but they died recently. At the end of October I went away for about 10 days, and came back to find 160+ nitrate and my tank covered in algae. At the time, the fish seemed fine, and I cleaned the tank and did water changes for a few consecutive days. I did not add any chemicals besides dechlorinator. In the following week, I woke up several mornings to a dead fish. (This is actually an I've had in the past with this and other tanks, that really needs its own post - mysterious fish deaths AFTER water changes.)

For the last month plus, the parameters have been stable.

 

[Slaphppy7]

@Colin_T , have a look

 

[Colin_T]

It's really hard to follow what has been written due to the varying size of the text.

Is the fish normally that red colour?
Did you have normal white light on the tank when you took the pictures?

Black patches on fish are bruises or chemical burns.

High nitrite or nitrate can harm fish.

If it is chemical burns, then water changes usually help. The black usually takes about a month to go.

 

[Oblio]

For reference, here is a healthy Glofish of what I think is the same color. Note: NOT the OPs fish or tank.

 

[EliK]

Sorry if the original post wasn't clear. Not sure how the formatting turned out that way.

The red glofish has always been the same shade of red, with a few greyish stripes and patches, and darker colored fins. It was never the same color shown by Oblio's post, which is more red-orange, rather it was always a deeper red and black mix.

What concerned me were the black spots under its mouth and behind its right gill. They seem to be significantly raised, like fungus or blisters on the skin of the fish. They appeared overnight.

I don't see how they could be chemical burns, since I do not add any chemicals besides dechlorinators to the tank, and have dedicated buckets which I use for my water changes. Also, none of the other fish have been affected. The parameters of the tank are normal, without high amounts of nitrates and no nitrites at all.