Stop fertilizing it and cut the time under the light gradually back to at least 8 hrs and then increase it as spring approaches.

It needs a cooler place and closer to the light. Low light and warm temps will cause it to get tall and skinny.
Mike

Thanks for the advice!

Follow up questions:

Do you think I should buy a brighter light? I have the incandescent bulb 12" away from the plant for heat concerns. Would a fluorescent bulb be better suited for trees?

Will the elongation cause any negative side-effects?

Thanks again,
Feed Me Seymore

They make a fluorescent bulb just for growing plants. A brand I used to use was called Gro-Lux and it worked just fine when placed right over the plants in my cool basement. Normal upstairs temps were too high for good sturdy growth. Too much elongation will result in a weak stemmed plant that is likely to fall over unless staked. Even then you're going to have problems. Keep the tree cool with plenty of light. I usually gave the plants twice as much day as night.
I wouldn't get any closer than twelve inches with a large incandescent bulb because of the heat, as you are aware.
I don't grow trees indoors anymore, I wait for Spring and do it outdoors. Less complicated that way.
Mike

This the new setup:

60 watt fluorescent bulb
2325 lumens
6500K
16 hours a day
Lowered the thermostat to 65*

We'll see how it fares.

FMS, I would stop the fert and really make sure i didnt over water.

Al