Anyone knows the cause of purple stems? And what c...

Hi @Yung_Ar9! Sometimes purple stems are strain-related, then you don't have to worry, it's just genetics. But if you notice other symptoms, such as brown spots or curly leaves, then it's a sign of a Phosphorous deficiency. Is your container perforated? Otherwise you may have a salts buildup, nutrients are agglutinated somewhere into the soil and your plant cannot absorb Phosphorous anymore. Over-watering can also lead to this nutrients-lockout issue. Roots need to be on a wet/dry cycle to properly develop and spread, searching for water. Do you let the soil dry a bit between each waterings? Also, close up / zoomed pictures could really help us diagnosing your plant's issue. I'll be around, keep us up-to-date and happy growing!