HThose are small for 3 weeks, I agree probably over watered. What are you growing in coco? Coco is almost impossible to over water may want to add perlite for aeration. What are you phing water to, where do you get your water from...tap/r/o. If tap if you don't use a filter made to remove chemicals like boogie blue do you let the tap sit for 24 hrs allowing the chlorine to evaporate.