Hello people of the forum!

I live in Luxembourg (where it can easily not be sunny for a couple days in a row). At the moment I have 2 baby apple trees which came from seeds that I planted.

They're only a little more than 3 months old and they've always been indoors. The weather is nice currently (not cold but not entirely hot all day) and sunny at least more than half of the day.

Recently and quite progressively, the leafs on the side been turning brownish.
Note; just is recent and at first they were doing great! Something change though..

Also, I water them every morning with tap water (usually once a day) which I let sit out for more than 24 hours before using. Soil seems to be decently moist (never really dry) and I've used potting mix (if you can tell) for the soil.

The leafs are progressively getting worse and I have no clue what's wrongl
Chlorosis, overwatering, underwatering, over fertilizering, too dry of air, salty water, or bad PH levels? Any ideas? :-(