I'm a newb too - so take my advice for what it's worth. Firstly, the Cotyldon leaves are packed with nutrients and they contain everything the seedling needs for the first couple of weeks at least. If you add nutes too early, you are overfeeding your babies and that may make them sick. By my very untrained eye, your seedling does not look all that healthy. Your seedling should look more like dimebong's picture. It definitely looks like you are over-watering. What happens when you over-water is that the soil turns to mud and you suffocate the roots. In my view, stick your finger in the soil up to your first knuckle.....if it feels moist, then no water is needed. You may want to take a spray bottle and lightly spray to keep the surface moist. When you water the soil should be moist but not soaked or drenched. If you pick up a handful of soil and smush it together by closing your fist, you should be able to easily break it apart with a finger on your tother hand (once you re-open your fist of course). That's my two cents and I hope you find it helpful.