Hi. I’m new to this hobby, and now I have a problem. Please tell me, why does my plant look like this? I thought it’s a light burn, but i give them only 10000 lux/40cm/18 hours.
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@kuujussi Welcome to the community 
nice to have you with us
a bit more information would be helpful
it does look like light/heat stress, but tell us all more about your environment
air temp, soil temp, etc it will help folks to help you
all the best and again welcome
Dequilo
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It cannot be heat stress for the temperature is between 18-22°C. Roots might be a bit cooler. I have not given any fertilizer yet, but earth is new and it’s" pre-fertilized". This is the 3rd week of growth.
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well there you go 
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Have you found the curling in the leaf to have gotten better or worse as it got to three weeks?
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Also how much are you watering and how often? The medium looks dense and wet. Is it drying out in between waterings?
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Heat stress is not the same as light stress and vice versa. 2 different things and it’s all in the name. This must be one the most common misunderstandings you hear in our growing culture. So many using the terms wrong or confusing one with another.
Heat stress = caused by temperature.
Light stress = caused by excessive levels of light. PAR. PPFD. So while your problem might not be coming from temperature stress, it still can be light stress. What is the amount of PAR they are under?
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PAR is now only about 100. For a couple of days I kept even lower. +I’m using Photone app.)
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I haven’t watered them for some days now. The medium 1/3 ceramic rock (i don’t know the word in English) and 2/3 organic soil.
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Leaves have been curling for two weeks, and getting brown tips later on.
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Is the soil still moist?
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Moist yes, wet no.
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Perhaps let the soil dry out, water with a certain amount each time, eg 450mls. I like to lift the pots and feel how heavy they are when dry and wet. Im not sure if that is your problem or if the soil is a little hot. Either way i would say your plants will come right as they grow.
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