The first leaves to sprout from seedlings will always die off. Usually around 2 weeks to a month. But the tips of your big leaves should not be brown.. sounds like a bit to much nuts.. Do you check the ph and pph of your soil/water? If you have over nuted by just a little, I would get distilled water from the market. Use this for 2 weeks to play it safe (no nuts). You should stick your finger in the soil.. If you feel any moisture in the top 2 inches you should definitley not be watering.. over nuting and over watering are a major problem with people who LOVE their plants to much : )

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