Hi mate. I think 1.5 kg is too much. Last time I used 200 grams in a 20 L pot (look at my green poison xl). Im currently in week 7, and despite the ec is high (oscillates between 1.6 and almost 3), the plant isnt showing signs of nute burn. However, ph sometimes goes up to 7.3. I have made a couple of flushing, so see if you could also do it.