It's hard to see in the photo, but clear when looking at the tank, that the new growth for this stem plant is growing with greatly reduced leaf size and reduced distance between nodes.
It's essentially growing like a dwarf version of itself.
I thought this was a sign of Nitrogen deficiency - but the tank has never had so much nitrogen.
About 8 weeks ago I started E.I. Dosing dry ferts.
My nitrogen went up to around 40ppm for a couple of weeks.
I've brought this back into balance and now it's around 25ppm.
So how can it be deficient?