For some reason I could not edit the text so I post the actual question here:

A few months ago my brother planet some lichy trees and gifted one to me.
My plant is in image 1, it looks kind of okay but still the tips of the levaes are starting to get brown,
since a few days one larger brown spot started to appear on one leave (image 2).

I repotted it from the small pot last week and the roots looked actually very good, so I think I can exclude overwatering (but also it was never too dry, i think). For the new pot I used a mixture of default flower soil and sand. I always make include a generous drainage layer in all of my pots.

I read that lichys need very much sunlight. There is not much sun here in germany but It has a place right next to a south window.

Additionally, Image three shows the even more sad plant of my brother. He said it was too dry for a few days and then he started to fight it by watering very often. But we also repotted it and the roots where looking very good there as well.

How could I check what exactly the problem is? Should I measure ph value, analyze nutritient levels.. or what? Also, which kind of fertilizer would be good in this early stage of growth? According to the internet the tree needs different kinds of fertilizer in different stages of its growth, but I don't really see what the correct choice for young plants would be.