Neon Tetra Help

[Harlequins]

Hi there

I noticed this morning that one of my neons appears to jerk through the water,it swims,then seems to have a jerky movement,was eating ok,no apparant signs of lumps,bumps or spots!
Water readings ammonia/nitrite zero,nitrate 12.5,ph 8.5(same from tap),last water change sunday.

Any ideas please?

Thanks