Do you know what the pH of your soil is? That might be the first place I'd look.

Dave, I'd add iron to the soil..You can buy Iron sulphate or Chelated Iron at your local nursery, hardware store or online. Adding iron should be done 3 times a yr as a preventative...Even though your fertilzer contains the essentials doesn't necessarily mean it's getting enough of one..very similiar to ppl who take multi-vitamins..many ppl take added vitamins like E..sometimes there's just not enough of one vitamin, therefore we take additionals..Toni

chlorotic from the sounds of it
what I do with my plants is to give a little bit of iron in the water, spray the leaves with it too
then get some RUSTY nails and stick them into the soil
the initial iron will start to take care of the problem, the rusty nails/screws will rust slowly over a course of time and release iron slowly into the soil keeping them green for many years

A pH test is essential if you want to maintain the plant's ability to take up iron (and other essential elements). Test first, THEN decide if you need to treat the soil. I sure wouldn't add iron sulfate unless I knew what the soil pH was!

A quick fix would be to apply a chelated iron as a spray. That 'might' get the plant in better shape, though it is a temperary fix. Please, see about that soil test.

I agree checking the pH is important, but by your desciption it sounds like an iron definciency. Pick up a pH meter...Rapitest.com sells a good variety..Toni

I did not realize I might need additional iron. I knew that citrus needed iron, but that is one of the trace elements listed on the fertiliser I use, so I thought it would be ok. The trees seem to be growing ok, plenty of leaves and new growth, just that a lot of them have that dark veined appearance. I will have to see about that soil test, although everything else seems to grow great in it. Thanks, Dave

Dave, most citrus foods contain trace elements, but some plants need an extra boost..Think of it this way..if you add the iron, it can't hurt..It's not like adding additional fertilzier which will kill a plant..
Some plants like citrus, azalea, garednia and hibiscus need extra iron, despite the fertilzer we use..Toni

An excess of iron creates an imbalance in the soil. It can even tie up other micro-nutrients, causing deficiencies in (for example) zinc and manganese.

I strongly urge that you contact your local extension service and arrange for a soil test. They will tell you how to collect the sample or samples and they'll send it off to the lab. The pH probes are notoriously inaccurate, and with some important citrus trees at stake, I'd sure go with a lab test. Florida extension still performs this service for VERY little money.

Once you've applied the fertilizers to your soil, you can't take it back. Makes good sense, doesn't it, to obtain some accurate information from the soil tests first?

Here is a link that might be useful: Click here!