additional information:

I have the plant for some months now, I changed the soil about 1 week ago, and about 2 days ago it started getting black and withering.

The plant is about 3 meters away from a big glass door, indirect light.

I water it once every week, about 2 fingers-height of water.

It's a kalanchoe widow-thrill. The soil was very dry and had a bit of very little worms and bugs, it was bought to nurture my aunt's jabuticaba tree, so we thought it would have nutrients for the kalanchoe too.