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Got this bonsai as a gift about two months ago and since then I've been watering it twice a week. Since 3 weeks ago some of its leaves are getting this brown/black spots on them, they'll get dry and fall after few days. I read in some other post that it may be a sun burn but my plant is indoors and not getting any direct sunlight. Also haven't used any fertilizer, it came with some green fertilizer mixed with its soil though, should I refresh those? Have not reported either.

Any ideas what it may be?

Thank you,