<span class="link_user">@MADMANk</span>, Thanks for all the help and tips :)
Will feed only pH adjusted water from now on and try to ride it out.
I will also have look at the roots after harvest.
Thanks also for the hint at enzymes, will have a look at that before the next run.
My suspicion is I didn't really take care to water correctly at the beginning (just using tap water, not adjusting the pH, sometimes watering to fast with either too little or too much drain etc.).
I will try to learn from this grow and gradually incorporate my learnings into the next ones :blush:
<span class="link_user">@flatcap</span>, just testing the run off wont actually be testing the mediums ph or ec range, you want to saturate a large amount of the medium over time to get a correct reading on its ec and pH level.