My 4 week old chili seedlings, various sorts, have started to develop weird almost see-through spots.
So as you can see the leaves have multiple small spots, where I assume no chlorophyll was developed or the cells died, and I am wondering if the plants just need some kind of fertilizer or something else.
First thing to check is if this is due to leaf miners. Commonly miners will create distinct tunnels, but in this case we see patches, not tunnels. Look carefully at all the patches to see if you can see evidence of larvae inside the leaf chewing at the interior layer of cells.
If that does not apply, the nature of the damage appears to be an over-richness of some element rather than a lack of it. Examine where your water comes from for levels of chlorine; consider using rainwater for example.
Probably the ph in your water is not good. Keep it between 6.0-6.8 for chilies and see how it changes your plant.
Also keep it always moist but drained
Tomato fertilizer is good for chilies
It could be thrip damage.
Key thrips damage symptoms
Sunken streaks and patches – leaves and stems
Colored spots or patches – usually silver-white
Stunted growth – buds (flower and leaf)
From Thrips damage and symptoms – how to tell if your plant is infected by thrips
Example image from above as well.
