Oreo

I use Advanced Nutrients and I'm okay with it. My question is does everybody use CalMag? Do you add it continually like any nutrient or just when you see yellow leaves? What's the story on CalMag?

Question: I grow indoors. Everybody says the room must be entirely devoid of light so I put duct tape on cracks and it's sealed. My question is, why is darkness so important when outdoor growers have the moon to light up their grow at night.