What can I do?

Might as well be phosphorus but yellowing of leaves is tipically not associated with phosphorus deficiencies though not unheard of. My experiences have shown that P deficiencies lead to dark spots and splotches on leafs and not a clean pattern like these are exhibiting. Clearly going from burnt serrations to yellow margins and then to green and I can see the serrations on the newer growth above it starting to burn aswell. P defs don’t do that. I still recommend starting on feed to avoid further compilation. Cheers 🍻

Hey there, bro! 👋 The plant is showing a slight phosphorus deficiency, the stems of the leaves are red. But these are the lower first leaves, so there's no need to worry about it at all, just keep an eye on the new growth, and those leaves die off for everyone at the beginning of flowering, so you're all good, my friend! Don't worry, be patient and keep growing! 👌😇👍 Good luck, bro!

If you have yet to fertilize and running on soil alone I wouldn’t worry about over fertilization at this point. I don’t see any burnt tips, clawing, glossy appearance. It’s easier to diagnose by looking at the whole plant but from what I can see she doesn’t presently look overwatered though her leafs stems are dark purple. They are definitely not just yellowing naturally from lack of light as the older bottom leaves in pictures have burnt serrations that transition into yellow margins and not yellowing evenly. Look up potassium deficiency and you’ll agree that this fits the description. Just make sure your ph is between 6.2-6.8 to rule out ph lock out and if so go ahead and apply preferably a balanced organic amendment

Hey Canadian, Thanks for your answer! This is not a plant from my grows shown. This plant is a Triple G and unfortunately it germinated 8 days after the other 2. In the meantime, I planted a critical in the same pot because I assumed that Triple G would not come. By the way, I use soil that is pre-fertilized and therefore I only water with water. About 200-250ml every day. So I don't know how to slow down the absorption of nutrients.

Hey Experimentgreen, Thanks for your answer! Yes, they are from the first leaves. It could be that I accidentally touched the leaves with nutrient water. And yes I only use Rhizza and Vega so far. The pH value is between 6.2-6.7.

Any chance that's one of the 1st 3 sets of leaves? Cause it's very common for those to yellow and die off. Otherwise it may have been low enough to where water, nutrient water splashes on it and burned the leaf surface. Especially since your ph on the other grows is 6.5 and your just running the rhizza and vega?

If this plans is one of the ones you are showing in your diary I'm going to say that probably it is exercise of nutrients because your leafs are showing a very dark green in that case is a clear indication of vegging nutrient-rich diet and it might be a indication to slow down on their diet.