Blätter hängen durch

It could be anything.Is the medium all bark or just a top dressing as its very dry,to much if that more than a top dressing.Is the pipe auto feeding.Need more info.

probably too much light but watering habits need to be reviewed too. 1) fully saturate (10% runoff minimum if soilless) 2) wait for safe dryback and repeat. anything over-complicated or deviating from that is watering wrong. if not doing this, could be a contributing factor. the top looks super dry, so i doubt it is related frequency of irrigation. If waiting too long, you may need to irrigate more frequently. 40cm is very close. Only a somewhat weak light with very well distributed diodes could do that and you'd probably need your environemnt on point (not too high of a VPD, etc). Are you also runnin gmore than 16-18 hours for vege or autoflower context? Too much light doesn't always immediately cause symptoms. You may have been giving too much for a week or 2 before it caught up to the plant. slow growth or internodes being short are symptoms, too. Is this some sort of hemp bucket? may need more oxygenation of the water, if so. not enough info... hopefully this stimulates some thoughts even if not specifically these things.

It's either under watering, over watering, or problem with too little room for the roots or too low temperature for the roots.

You need to start a diary and fill out all the information you can............without a diary and not knowing what you might be or might not be doing, answering questions accurately, based on one picture only, is almost impossible.

Looks like to much water, maybe the roots got an issue💚