My indoor lime tree is developing a yellowing of the leaves, and looks rather unhealthy.

How do I correct whatever is wrong?

If the yellowing is on the older leaves of the plant then it is due to magnesium shortage.

If on the youngest leaves this would indicate an iron deficiency.

A good tomato food should contain enough magnesium & iron to correct the problem, but in a severe cases, spray the leaves with 20gms (¾oz) Epsom Salts to a litre of water.