Lemon Tetra with red sores

[ColdVendetta]

I just picked up 10 lemon tetra. I acclimated them etc. One had a reddish sore on his side. The red (light red) sore appeared to be raised. I put them in the tank around 10:00pm and my midnight it had died. I wake up to find the next morning one more has died. I returned them and got two more last night. One of the new ones has a similar sore on his head (raised and red). This morning a different one was dead. The one with the sore on his head is alive but the sore seems to be getting worse. He seems to be slightly gasping (the first one that died was gasping near the bottom as soon as i but him in the tank).

There does not appear to be spots on any of the fish.

Also in the tank (55gal) are

4 assorted angels (nickle sized i picked up last week)
1 black molly
1 silver molly
(the mollies i picked up same time as the angels.
All the other fish appear to be fine

Ammonia 0
Nitrite 0
Nitrate 20
Temp 79
pH 8

What is this, and how do i treat it. Should i stay away from copper based meds?

[ColdVendetta]

Does anyone know what is causing this? I just lost one more and a couple others are not looking so good. They keep developing these raised pink areas on their bodies.

[Bruna75]

I don't know what it is, but I had a few rummy nose tetras that died right after developing red sores out of nowhere:/ have you had any more deaths or found out what it was yet?