I have a nice little Ficus tree that was doing well in the potting soil. I switched to something called "universal bonsai soil" Now it's dropping leaves quickly, 3 or 4 a day. I have another one that's doing well in the same soil. Where did I go wrong?

Impossible to answer based on the given information.

How long have you had the tree? How long experience in bonsai? How much root pruning did you do? What is the location of the tree indoors or outdoors? Where are you and what are your climate like right now? How often, what way and how much are you watering? What is a brussels ficus? Never heard på that species. No ficus growing in brussels as far as I know.

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