Hello,
Thank you for replying.
I forgot to specify that it is an indoor bonsai, so I thought that it is actually to be kept indoors.
Thank you for specification about the tray. When I was watering, the water would come out the drainage hole and onto the tray, that's why I believe it was overwatering that did the deed.
There are not one tree in the entire world that grows naturally indoors. Indoors is a term used by the industry for trees that needs some protection in winter. They are produced in greenhouses.
But indoors dont kill that fast. Its more a slow decrease in health and the death normally occures in winter time when it is far too hot, too dry and lack of light.
What killed your tree was underwatering. A bonsai should be watered abundantly over the whole surface until the water pours out in a steady stream from the drainage holes. And that is not easy indoors.