What might cause this?

[damnlemon]

any idea?

[brian]

Sunburn?   Where do you live?

[damnlemon]

no, I keep them inside

[Laaz]

Salt build up.

[lebmung]

is that a key lime? looks like a seedling

[Millet]

I think Laaz is correct, in that the leaf damage is due to soluble salts in the medium.  Soluble salts come from the water you are using, or from the fertilizer being used.  You can wash the soluble salts out of the root zone by flushing the container with distilled water.  Pour a gallon through the container and let it drain out the bottom, doing which will flush all the salts out.