Ok- well, my guess about the nitrogen is uncertain. It could be just overwatering causing the downward pointing look of the leaves, along with the lighting in the photos making them look dark green.
Or - there could be nutrients in whatever type of soil you are growing in causing trouble. Worth checking that out.
Re the watering thing- cannabis does well when it has a chance to dry up between waterings. And especially when they are small- they should never be very wet.
The best way by far to tell when they need watering is by testing the weight of the container.
Fill a solo cup full of dry soil, or soil that is just very slightly moist- like the bare minimum of moisture. Compare the weight of that container to the one with the plant in it. That's what you should be aiming for when they are small like this. When they get larger you can soak them down, but at this age you have to baby them a little. Overwatering is the number one killer of seedlings.
Here's a great link full of growing info. Scroll down to the watering section and check the 'lift the pot method' thread. You'll probably find some other interesting info along the way.
How to Grow Marijuana Everything You Need to Know