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If your only seeing a random flashing, it could a some type of behavior. Once in a while I see one of my TigerBarbs flash on a plant, a few seconds later a different one flash with no illnesses.
Usually fish that flash due to ick will flash on a rock, substrate or DW.
It may just be behavior... What is the new fish? That could help someone else with that fish give input. You mention you have a longline Abn with them.