I don't know why out Grapefruit tree has yellow leaves. We planted it two years ago and it was doing well for a while, but later got worse. We water it regularly and I put some citrus fertilizer about six months ago, but it did not improve it. Two years ago it had two fruits. Since then, it failed to make any other fruits. The yellow spots on the leaves are quite characteristic, I hope some experts will provide some details about what needs to be done. Maybe at some stage I put too much fertilizer? When I planted it, I didn't put any fertilizer, because the tree was in very good condition. At the time, I dug a big hole in the ground and we put good quality soil. But the soil around the tree is pretty bad. It's some sort of clay and water does not penetrate. After checking some diseases in citrus trees online, I suspected there is a magnesium deficiency. I dissolved about a tea spoon or a spoon of Epsom Salts (Magnesium Sulfate) in water and watered the tree a few months ago, but I didn't notice any improvement. Maybe I didn't put enough? I try to avoid over-fertilizing it, because it is a small tree.
I uploaded a short video on youtube, I hope it shows better the leaves. Some of them appear attacked by some insects. We live in Sydney, Australia.

Regards,
Nick

http://www.youtube.com/watch?v=IuzFfsCXG2s