could also be sunburn or light burn (too close to a hot light)

I fertilized about over a month ago. I didn't think the light was too close but maybe? How close should it be?

If it's salt buildup how to get rid of the salt? Is there a test to measure the salt build up ? Thx for the help!

The tree gets maybe a couple of hours. I just moved it where it will get more light

The leaf drop is most likely Winter Leaf Drop (WLD) due to cold roots and leaves getting a lot of light. See: http://tropicalfruitforum.com/index.php?topic=13332.0

Also, in addition to WLD, your soil looks to be too water retentive which means you may have root rot due to wet soil. Moisture meters are not very accurate. You are better off relying on your fingers. Dig about two inches into the soil and feel if it is moist. Or, stick a skewer to the bottom of the pot an pull it out. If you see soil particles sticking to it that means the soil is moist and the plant does not need watering. If you do have root rot, you should repot into 5:1:1 or gritty mix. Here is the recipe of the 5:1:1:

5 parts pine bark mulch (remove ½” pieces through 1/2 inch screen)

(Some people use Repti Bark instead of mulch. They do not screen it.) Try to soak the bark overnight.

· 1 part sphagnum peat

· 1 part perlite (coarse, if you can get it)

· 1 Tb garden lime /gal 5-1-1 mix (measure the 5-1-1 mix)

· Osmocote plus (follow manufacturer’s instructions). This is optional.

· Moisten well and wait at 2 days before using, if possible.

If it is salt, fill the pot with water and let drain. Repeat three times.

Yes, once that tree needs watering, pour fresh water through it and let it run out from the bottom at least a minute...Do as Vladimir says..It definitely looks like salt damage..Your mix is holding a lot of salts from tap water and fertilizers because it acts like a filter and absorbs the salts instead of ridding them. There for the need for a good porous mix)