What's causing the leaves to lighten?

Hey man, It's clear that this is the beginning of a nutrient burn due to excess nutrients. Be careful in the first few days; if your substrate is rich, it's unnecessary and even dangerous to give too many nutrients at this age since the roots are fragile. You risk having the opposite effect because too many nutrients can burn them at worst, or at least slow down growth. Personally, when I use enriched substrates, I make a pocket of lightmix at the seedling stage to avoid this kind of reaction. Wishing you a great and wonderful 2021 filled with lots of greenery and flowers. 👉🤩🌱🌿🎍👨‍🌾😁😁

Too many nutrients at this stage of growth. Growing in ffof, you don't EVER add anything else for at least 3 weeks, that soil runs hot as it is and adding additional nutrients is just going to cause problems. Ph'd water only for a while until she tells you she is hungry. Happy growing.

Hey Bro, The Fox Farm Soil is heavily pre-fertilized, isn't it? Looks like a combi of too much water (wet soil) and starting nute-burn. Keep going with pH'd water, think it's the best way. Decrease the amount of water when top feeding. Go for a decent shower everytime shortly before the lights go out. When they are small they tend to drink more via its leafes than via a not fully developed root-network. Good luck and happy trails for the rest of the way🤞.. Servus OimRausch

Don’t give her cal-mag at this stage! Burns the plants. I got the same problems before. This cal-mag product is very strong.