Clementine Tree/Plant help

I got given a clementine tree / plant and when I got it the leaves were a dark green, now a light green. I have recently repotted it but no change as yet. I have all so brought some citrus food. What am I doing wrong?

 

I find citrus to be very tricky plants to grow in pots.
They tend to suffer from all sorts of deficiencies including too wet or too dry.

All of which can make the leaves yellow or fall.

If you have re potted it, it may well start to look up in a week or two.
If not it could be magnesium deficiency so a couple of waterings with epsom salts might green it up.