Well the first thing we need is more info. Test your water, check everything is working ok and do a 50% or so water change with dechlorinated water in case it is a filter issue.
It could be Neon tetra disease if there is a loss of colour, but until we know that everything else is fine then that that would be speculation.
It could be Neon tetra disease if there is a loss of colour, but until we know that everything else is fine then that that would be speculation.