For one thing, it looks like your kumquat is being attacked by leaf miner. Get some spinosad spray, and thoroughly spray (making sure to get the undersides of all the leaves) every 10 days, 4 different times.

Looks like a post harvest Nagami kumquat; they are heavy producers and heavy feeders. I would give it a good shot of a good citrus fertilizer, applied at the drip line of the tree and watered in deeply. I have a seedless Nagami at my lemon farm which produces constantly; and although some people say they like them, nobody eats them. Last week I made a kumquat marmalade, which turned out pretty good; I gave it to a local chef who is known for creative energies.

What mix is your tree potted in? How long has it been in that mix. None of my kumquats show deficiencies and I feed them the Peters fertilizer, Oscomote Plus, and FP once in a while. I have no issues whatsoever. I had a tree that I adopted from my neighbor that had been potted in potting soil for a year plus. It showed deficiencies regardless of what I did. A few months ago, I removed all the potting soil and put it in 5-1-1. Although the old leaves are not "corrected", the new growth looks healthy. You may not be able to fully correct those older leaves, but if you start feeding it correctly and put it in a good mix, the new growth may be normal, which a good indicator of the tree's current health. Best of luck!

I would disagree that you can't fertilize with organic fertilizers in pots, however if you are going to do so you need to be diligent about how you care for your tree, use a medium that is appropriate for and encourages the growth of microorganisms, and avoid plastic pots. You also have to be careful not to use anything that can damage your microorganisms. When you use organic growing methods in containers you absolutely have to treat your soil as, as much of an organism as your tree, and I would not recommend it to someone starting out with citrus. (I grow my own trees indoors in a huge window exclusively under lights and use organic methods. I get fruit every year and my trees grow very well.) Reading your responses it definitely sounds like you have a few problems: water, light, potential overfertilization. You don't "feed" a tree the way you would feed a pet, fertilizer is not "food" in that way, but should instead be applied as necessary to replace nutrients depleted from your soil. Indoors, your trees won't deplete nutrients as quickly due to the low light and slower growth, so you don't need to fertilize as often or at all. As for watering, as mentioned above you should let the tree dry out between deep waterings and watch that your soil doesn't stay wet for too long. (For example: self watering planters are death for many indoor plants). As for lamps, the kind of grow-bulbs or nice looking growing lamps are not going to provide you enough lumens to grow a tree. You could probably use them for little plants that require low light, but a tree needs more than that. A high powered LED lamp is going to be your best bet for a citrus tree. There are options for ones you can install in your house or you could hide them in some way, but if you want your trees to do well you need a lot more light than a standard grow light. I hope this was helpful, and good luck.