Thanks for your answer!

Please note: The twistet leaves under the tops are from an accident about 3 weeks ago (sprayed with a very sour bio-defense water by mistake but this is not the reason for the yellow leaves)

Here the troubleshooting form:

What is your experience level? (first timer, novice, experienced...)
Novice

Your Equipment:
A) Type and wattage of lights. (MH, HPS, CFL's, tube fluorescents, LED's)
400 w HPS (red light now)

B) Distance from tops?
About 13 inch (35 cm)

C) Reflector type? (cool tube set-up, bat wing, enclosed reflector, bare bulb...)
Sputnik air cool reflector (like cool shade)

E) Is there a consistent fresh air supply?
Yes, good air exchange!

F) Do you have an exhaust fan and a circulation fan?
Yes

G) Size of reservoir?
10 L. (2,64 gal.?)

H) What medium if applicable.
Biobizz Allmix

Your nutrients and water:

J) Source of water. (tap or filtered) What's it's ph?
Tap-water, ph 6,2 (rectified with phosphoric acid)

K) Specific brand and N-P-K ratio for each bottle. List dosages (quantity per gallon) and current feeding schedule.
Complete Biobizz nutrients (dosage a little bit under the Biobizz schedule)

Q) Do you foliar feed? If so, with what, how often, and at what time do you
spray? (Just after lights come on, just before they go out...)
No, only B´cuzz bio-defense once a week, 5 min. before lights come down

Your growroom:
S) What size of closet, room or hut?
Homebox S 80x80x160 cm (31,5x31,5x63 inch)

T) What are the temps and humidity levels while lights are on? ...With lights off?
On: 77 degrees f. - 84 f., hum. 35-50%
Off: 73 f. - 75 f., hum. 35-50%U)

Have you seen signs of insects in the growroom?
No

V) Are the roots long and white, or brown and slimy?
Normal white and very healthy

Your strain:
W) What strain are you growing? (Indica dominate or Sativa dom?)
Bubblelicious (mostly Indica)

X) From seeds or clones?
Seeds

Y) Is this an autoflower strain?
No