you can buy ferts made for that, usually micro
nutrients,
but you must make sure you get a wetting agent because as you know plants don't feed in the dark so they must be sprayed in the light, the wetting agent lets the fert. absorb into the leaves quickly with out burning,
i have had good luck with coco wet and the fert that goes with it
side note: don't spray seedlings, i learned the hard way....