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What are your water parameters currently? How often are they flashing? It's not uncommon for fish to flash occasionaly but if theres more frequency then something is wrong.
Keep an eye out for ICK.
It can affect the inner layers of the gill without signs on the exterior.
I was puzzled by it for 2 weeks once. I lost 5 fish before I saw exterior signs on 1 other species in the tank. I started heat and salt treatment and that solved it.