Any ideas?

[Hybridfish7]

Have you tested your water?

Yes

If I did not test my water...

1. ...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

If I do not change my water...

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

Just got a batch of pearl danios going into quarantine. Trying to figure out what this is so I can treat accordingly/get the others away from it if necessary. The lesion is symmetrical on both sides, appears two dimensional/under the scales for the most part. Other than that fish acts normal. All the others are not symptomatic but of course they were in the same store tank/now in the same bag so I am treating them all the same for now. Picture was not taken in the quarantine tank, was just floating them in another tank while I got it ready. Praying I didn't doom the other tank by floating the (dry) bag in it.

 

[Hybridfish7]

Small update, lfs owner took the sick one back. Just going to observe the good ones for a few weeks.

 

[danotaylor]

I’m glad you qt’d the rest mate. I’d give them a solid 8-10 weeks of observation before adding them to any other tank and be sure not to share equipment.
Looked like a pretty nasty bacterial infection, so I would add salt @ 1tbsp/2gal initially and redose to keep that concentration with water changes for a couple weeks to break the reproductive cycle and kill off the bacteria the others may be carrying.