Dying barbuses and neons

Question
I have a 250 liter community tank with excellent plants which has been going for years. For past few months all my neon tetras and tiger barbs are dying with reddish sores on the body when I can get them before they 'disappear'. I am sure it's not Velvet. Other species are unaffected. I am a pediatrician myself but have been unable to diagnose or treat this disease. Any help gratefully received.

Answer
I agree with you that it isn't Velvet. Velvet just doesn't do that.
It could be Red Pest which is treatable with some antibiotics that can be found at Pet stores.
It also could be Tuberculosis which causes a hollow belly. Best to destroy the fish.
Ichthyosporidium could also be the culprit of this. It causes hollow belly, sluggishness, loss of balance and external sores. Again antibiotics are the answer.

Hope this helps.