One plant looking weak - yellowing leaves with yellow vain

Hi guys,

Got a slight problem,

Leafs are turning bright green instead of a nice green,

The vain of the leaves are yellow in the middle.

Also lots of leaves at the bottom are yellow and falling (much more than other plants)

Pic attached.

Many thanks.

Is that under a hps light if so move it out of the light and check the plant. The hps light can be tricksters. Also, is there enough light to reach the bottom? Cause I have some plants they have a couple of branches that dont get light any more so they turn kinda like a neon green color.

magnesium issue? add a pinch of epsom salt and see if it stops acting up