Neons losing scales??

A few of my Neon Tetra's look like they're losing their scales.

Any idea what could cause this and how to help? I haven't noticed any behavioral changes, they're not scratching the (rounded) gravel or anything. No predators in the tank either and watertests are all normal. TIA!

A few of my Neon Tetra's look like they're losing their scales.

Any idea what could cause this and how to help? I haven't noticed any behavioral changes, they're not scratching the (rounded) gravel or anything. No predators in the tank either and watertests are all normal. TIA!