Ficus Bonsai leaves yellowing and falling off

[AnnaIkona]

Assuming this is a fungus, I sprayed the leaves with baking soda water, but it did not help.
Right now, the tree is on a windowsill, getting plenty of sunlight. It is also gently watered twice every week or so.
It is still in the pot and soil that I got along with the bonsai at the nursery.

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My previous bonsais have had this problem, but it somehow just went away. But this one I'm a bit concerned because half of all the leaves dropped already.

Any help is really appreciated!