Que me recomiendan para plantas en esta etapa

So you have this 3 inches seedling, with only one set of leaves, and you say your plant is over a month old? Sadly you have some weak genetic here. I can tell that this plat received everything she needed so far: light is good, distance from the light is good, she had enough access to nutrients in your soil, ph seems fine... She looks perfect and healthy. I am not a fan of those expensive bottle nutrient, and I believe supplements are a gimmick, not necessary. I am a proven organic grower. You could go on and spend more time and money on her, but I would pull the plug and call it bad genetic. Again, if you had not gave her everything she needed we would see signs: over stretch, yellowing, wrinkled leaves... none of them are visible. Problem is not about what you did, it's about the plant.

Great growth comes from healthy roots. Use a bioenhancer with your nutrients which contain beneficial bacteria, humic acids and kelp extract. All will help keep your substrate healthy. Water pH 6.2 when the top few inches are dry. Allowing more drying time as the plant grows due to root system. Cannabis loves a lot of light but can burn in the early stages if too intense. 50% intensity and 40cms height is a good start and change depending on how she responds. Try not to over water, keep humidity high, around 60%. Temps around 21'C, Increasing to 25'C/50%RH in flower. I hope this helps in some way, good luck with your grow