Hi
@Blinkin! You named it --> overwatering. You must follow a dry/wet cycle for the roots to spread, searching for water. Each phase should be pretty quick, the medium should not be dry for too long neither than wet for too long. Now as a temporary solution, you can put a tiny fan blowing a slight continuous breeze on your containers (not on the plants). A simple rule to prevent this issue is to water with only 15-20% of your container's capacity, unless you need a big drain / unless you want to flush the medium. Using a 7L container, you should water with ~1L. Water less, more often, watch for the dry/wet cycle and you should be fine. Hope this will help, keep us up-to-date and happy growing 👊