@gunkle: I understand your point, but the humidity in the terrarium falls down really fast due to the heatpanel and the material so there isn't enough time for the entire terrarium to become constantly humid. The only problem is the ground either because I accidentally mist too much or Kahlua spills his water dish.

@distaff: do you maybe have a link to this product? Well, it WASN'T and ISN'T as bad as it sounds. I ofcourse did something to the mold before I started the painting project. I killed the mold and let it dry for weeks after that I used sandpaper and scratched all the parts that didn't look brown off. So, I'm quite sure that there're no mold on the bottom anymore.
I don't know I can give you a link of his video if you want to, but it's in German 😅.

But I won't worry much about the terrarium doesn't last long anyway, fortunately, we're planning to build him a better terrarium sooner or later. Thank you guys for all the helps though. )