@InspireMe and gmslave,
thanks for your replies. I moved today the lamp at 48cm distance. The tent is really small in height 160cm. I think i'll have to move the carbonfilter outside the tent to gain a little more room.
Today i also watered with 1,6l for 4 plants at pH 6,4 with Cann Rhizotonic,Vega and BioBoost. I used 25% of the recommended dosage according to RQS. Added 0,2ml CalMag too.
I moved the 15cm vent a little bit more downward so the airstream is closer to the tips. I plan to add another fan which i already have.