One thing I have come to notice is that these trees seem to periodically have leaf sets that yellow and drop periodically. I'm starting to believe it is not really to stress but that the tree sheds its old leaves to grow younger ones with more vitality.
That's right, the BRT will generally drop it's leaves to push new ones out, but if you're strictly growing indoors under lights, as I am, your BRT may not experience this. And therein lies the conundrum. If you give your BRT 12 hours of grow light a day, it's always summer indoors, but your BRT can still sense when winter is coming or near or here. Indoor growing under lights could mean your BRT will drop some leaves, but not all, or no leaves at all.
Set the BRT In front of a window though, and chances are you'll see it shed the old for the new.
My BRT is a big one in an 8 inch pot and she's shed some leaves, mostly from cold damage during shipping to my zone, and there's some new baby growth but not the kind of growth I see from other bonsai (my Chinese Elm, for example).
There are a few yellow tinged leaves down low, some prune-ish looking ones, and quite a bit of healthy looking ones.
But it's January and it's raining ice outside, so all things considered...
It's good to get into the habit of checking your BRT for pests once a week. Inspect the undersides of the leaves, check the branches, and look for anything irregular. BRT's are fairly pest free but it pays to look. Sometimes, when all else fails, it's some creepy crawler at work.
As for watering, mine likes to be evenly moist but not completely dry. If you're unsure, buy a moisture meter until you get the hang of it. Mine tends to go 4-5 days with moisture before the lights dry her out.
Need indoor growing tips? Ask me!