Neon tetra disease, it happens. I've only seen it twice out of about 30 neons and green neons. Can't be treated and once it starts best to remove affected fish, can be spread through consumption of affected fish. When buying tetras avoid any disfigured or funky looking fish. Keep water parameters in check. Ntd aside there is the random neon death thing, they'll just die for no apparent reason.. happens..
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