They love a good feed of well-rotted chook poo late winter/early spring, and mulch well, though not right up to the trunk. Does it get splashed much from the pool? Maybe it's the salt levels and other chemicals. If it's not too big, you could try relocating it this winter, before the spring bud swell. Prune it a little beforehand. If it's too big, take some cuttings and try to strike them. It can be done. Failing that, buy a new tree LOL!

Well once again I agree with Ray!
It sounds like a chorosis which is due to a deficiency (possibly iron, possibly manganese) Anyway it's often not an absolute deficiency, rather that the pH is wrong, if the soil is too alkaline the tree may not be able to absorb the correct balance of minerals.
So check the pH (I use the one from my pool kit!) correct if necessary, then add chook poo! Or move it as Ray suggests.
Also make sure there is no grass growing around the base, citrus roots are quite shallow & they compete directly with grass for water & nutrients.
Good Luck!

I'd check the soil pH, too. I did with my tree and it needed adjusting. I also wonder if you're getting some sort of ill effects from the salt-water pool, perhaps via evaporation. Subject to how large and how old the tree is, transplanting isn't out of the question, but it's best done in Spring.

I had a lemon which gave me fruit the first year and nothing since. This year I cut it to the bone, and forgot about it. I topped up the soil with stuff from a nursery, and it's done a bloody Lazarus on me. I've never seen a healthier tree. Like the others say, the roots are VERY shallow, and are extremely susceptible to drying out and leeching of anything. They also get sunburnt quite easily. Maybe you could try throwing a bit os soil round the ground underneath it. It worked a treat for me. '
And I love the heading.