my grapefruit tree leaves turn yellow with brown marks
My grapefruit tree leaves turn yellow with brown marks. It is not very young tree, but we never have a lot of fruit from it. There's always something wrong - yellow leaves, small fruit, etc.
With the history of poor performance with your tree, I would highly recommend a soil test. You then can determine the correct fertilizing schedule.
Here is a link to help you locate your nearest extension service.
Also some great links about grapefruit tree care.