Red Marks Appearing - Gills, Mouth, Edges of Fish

[punman]

Have you tested your water?

Yes

If yes, what is your ammonia?

0

If yes, what is your nitrite?

0

If yes, what is your nitrate?

10

If I did not test my water...

...I recognize that I will likely be asked to do a test, and that water tests are critical for solving freshwater health problems.

Do you do water changes?

Yes

What percentage of water do you change?

31-40%

How frequently do you change your water?

Every week

If I do not change my water...

...I recognize that I will likely be recommended to do a water change, and water changes are critical for preventing future freshwater health problems.

These are juvenile Venustus. Had them three months. Eating fine and getting along. Just noticed this the other day on two of the five.
What causes it? Name of disease? Will it get worse and infect all fish? Is it treatable?