Hey All !!!!! :thumbsup:

Nikolaus please understand I am no expert, I leave that to stinky and rusty.

I do have some input that may help.

First your soil looks like clay or mix without much organic matter in it. Is it
a namebrand soil? If it is void of organic matter it won't hold water and you will have to overwater it. Watering 2 times a day is way toooo much.
I suggest getting a good brand of soil with loads of organics in it. this should help to retain moisture in the soil. Add some peralite to the mix also. I use Fox Farm Ocean Forest with perlite added with good results. I need to water about every 3 days.

Next I would move the 400w hps up a foot. A 400w bulb a foot from plant will get pretty hot. Hold your hand between the plant and the light see if your hand gets hot, if it does its tooo close to the plant.

You shouldn't have to have a fan on High blowing on the plant. This will aid in water evaporation and make overwatering a need. Just circulate the air around the plant with a good supply of fresh air coming in and this should help.

These are just my thoughts use as needed.

Good Luck