I received this Bonsai Ficus as a gift about 2 1/2 years ago. It has been doing very well. I watered it once a week (which is about how long the soil it came with took to dry) and made sure it drained well (there are 5 drainage holes in the bottom of the pot). We moved about 7 months ago, and the plant now sits somewhere where it gets the same amount of sunlight it used to - maybe less. About 2 weeks ago I noticed that some of the lower leaves were turning yellow. I do not prune it (not something I feel I know enough about to do) and so figured maybe it was getting ride of excess leaves. I let them fall off and it seemed fine. I then decided it was a good time to repot (a rookie mistake I'm sure) since it was showing some new growth and several forums on this site and other places online recommended early spring and about every 2 years, so I made a mixture of 2/3 aquarium gravel and 1/3 regular potting soil and mixed in some fertilizer and then followed what seemed to be the basic instructions for repotting. I combed out all the roots, washed off all the old soil (which also had fertilizer) and put it back in the same pot with the new mixture of soil and watered it. I tried to do a little root pruning, but there didn't seem to be very many roots in the first place, and I didn't know what I was doing, so I very much erred on the side of too little (it certainly was NOT root bound). I did trim off some of one of the larger roots that seemed black and unhealthy, but it wasn't much at all. For several days the plant looked fine, and I noticed that the soil drained and dried out MUCH faster.

However, yesterday, the remaining lower leaves have begun to turn yellow, and the formerly healthy upper leaves are dry and curling. Several of the leaves have brown tips but most of them are just yellowing.

What is wrong with my plant??