There's sticks and shit in that soil. Sticks are usually not a good sign...
Let's cover what you are seeing, then we can address the causes.
The first set of leaves, the oldest and largest, are bleaching out from the tips to the center. The symptoms appear to be spreading to the upper most leaves.
The veins between leaves are still green but the meat between the leaves is what has lost color.
There is very little, if any, necrotic (brown crispy) damage to the leaves.
For discussions sake then let us cover the suggested problems.
Nutrient Burn: Generally characterized by the immediate burning of the leaves, starting on the tips and extending along the edges of the leaf before consuming the entire thing. Burning is the immediate necrosis of the leaves. They become brown and crunchy.
Your plants are not brown and crunchy. This is why I do not believe this to be nutrient burn.
pH imbalance: An imbalance in the pH of the media will cause certain elements to bond which will create salts and make these element unavailable to the plant. In addition, the electrical equilibrium of the media determines the cation exchange capacity between the root cells and the solution of water and elements in the media (in this case soil). Basically, the decreased presence of electrical charges (free elements) can cause the root cells to lose elements in an attempt to establish an electro-chemical equilibrium in the media. This stunts root growth and can cause both deficiencies and toxicities in the plant. This makes pH problems both very common and very difficult to properly diagnose.
You plant is young enough to not depend on the uptake of nutrients, but if the pH of the water it is taking in is too low this could be causing the problems you are seeing. Figure if the plant is 80% water, and the water it gets is the wrong pH, then the elements in solution IN THE PLANT are going to lock up. I think the lack of vigor of these plants, the greenness of the veins, and the loss of chlorophyll indicates a low pH and a loss of Magnesium availability. Magnesium is the most crucial element to chlorophyll.
In addition to the pH problem I think over watering is a likely suspect. I'm not sure how you are determining when to water but what I find works best is watering by weight. Plants your size need VERY little water, you'll lose more to evaporation at this point. Lift the cup up when it is wet and feel the weight. Then allow it to dry out to a point where when you pick it up it has nearly no weight. I'm talking very light, it'll almost scare you. Obviously, if the soil is dry for too long the plant will wilt so what you're trying to do is find that point where the plant is just ready to really take a big drink but before it suffers. Then when you add water you want to add enough water at a pH of 6.5 for 1/2 of it to drain out the bottom of the cup.
This will ensure that the problem does not continue up the plant. You will probably need to begin using an organic nutrient pretty soon after this. You'll want to start light and increase dosage every time.
