I've had this problem a little while, thought it was nute burn at first as it happened after I upped it's feed. Not sure if it's a deficiency though now. Growing autos in tropic mix, feed doses were 1ml - 1 1/2ml - 2ml - 1ml per litre of water, had 4 litres each time except when I put it up to 2ml/1ltr then it had 5 ltr that feed, upped it each time then dropped it down to 1ml after tips were getting burned
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