Trees develop roots for a reason. They like to stay put.
Don't keep the soil wet, let it dry somewhat between waterings. The soil should be dry for the first inch or so, then just moist underneath. You can stick a wooden chopstick into the soil, all the way to the bottom, and pull it up in a few minutes. If it's wet, don't water.
Roots that sit in water constantly, are prone to rotting. They can't handle it. Chinese elms are fine drying out a bit.... never completely dry, but somewhat dry.
Outside would be better, but if you must keep them inside, try not to move them.
You should also check for bugs.
Finally, there are some Chinese elms that seem to drop their old leaves all at once, when the new buds push out. It is very possible that this is your situation, but being careful not to overwater, and checking for bugs, will ensure that the tree is healthy going into the future.
If you want to feed a bonsai, organics is a good choice. Search this forum for organic mixes. If you must use chemical ferts (I use them, but would prefer to use organics) then don't immerse the pot into the fertilized water. You need to pour the fertilized water over the pot and let it drain out. Otherwise, you very likely could give it too much fert. What NPK was the fertilizer that you used?
Good luck!
Joanie