Hi, 

You can try to send a photo of the entire plant(s) which may give is a better idea on conditions. We are suspecting that this is anthracnose, which is a fungal disease common on many types of trees. As the leaves fade and fall off, you can just make sure to dispose of them to help remove the fungal spores. They should recover and push out new growth,  assuming you are watering appropriately for the bonsai. 

Let us know if we can help further. 

Emily 

oh no- sorry for any confusion. You can attach them right to the reply.

The pot size looks fine and as long as you're watering as needed that should not be affecting the leaves. 

Watering, in summer at least, tends to be needed daily in small containers like that, and given that degree of flushing with water, a time-release fertilizer used in the potting mix is almost a necessity, or else you're looking at eventual nutrient deficiencies.

Maybe their location in what looks like a stairwell and its reduced air circulation is increasing their vulnerability to infection for anthracnose.

If you want to try a fungicide (if they put out new growth, since there's no point in treating what's already infected) that may be helpful. You can look for a fungicide labeled for trees and just read the label directions. It will help with the spread of the infection on new leaves but not cure existing leaf conditions. But as I said earlier, it shouldn't harm the overall health of the plant. 

Emily