It could be internal Hemerage.. internal bleeding.., I had fish like that my gold barbs... I was unable to treat it and eventually they died.. I cant remember the name of the disease that causes it.. look it up on google.
Have you double checked your ammonia level? Redness in the gills can be ammonia poisoning... Maybe a little mini cycle from switching out your gravel?
I'm curious to know if your ammonia spiked, I'm changing out my substrate from gravel to sand either today or tomorrow and have been worried about that happening.
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