Hmm this is helpful. I'm wondering if it is just the winter. We lived in New Orleans (roughly 10-15C in winter) and now live in Upstate New York (roughly 0C in winter). It is definitely less humid in our apartment than it was before we moved and we have to have the heat on all the time (We also have a reptile pet who needs it to be pretty warm).

We will try giving it main tap water rather than the brita (although it has always received brita filtered water in the past and never had any issues). We can try reducing the light too, but it doesn't get a lot of sunlight because our new apartment doesn't have big windows. We've always used this light in the past as well. We've had the tree for 8 years so the water and the light aren't variables that have changed recently anyhow.

I did just find a few small webs in the tree, but still haven't seen a single mite.

Brita removes chlorine and some metals, like calcium. I don't know about upstate New York but in the southeast US the water has a lot of calcium. I run water through a Brita filter for the tropical fish and they do fine.

The problem may very well be related to humidity. I used to water the indoor trees once a day. For Christmas my lovely wife gave me a soil humidity meter and I have been surprised at how quickly the soil goes from wet to dry.

The grow light shining on the trees is on 12 hours and they are doing better than any winter in the past, so I don't see a problem with 13 hours. I change water in the fish tanks regularly, save the old water, and use it on the trees.

If convenient, try putting the tree in the bathtub and turning the shower on it, soaking the soil and washing the leaves. Or, in the kitchen sink and use the sprinkler.

Where in upstate New York are you? Near Rochester? International Bonsai is there. I don't remember any indoor bonsai there but Bill Valvanis is always helpful.