First, your plants are stretching. The light needs to be within inches of the tops of the plants and a huge challenge for indoor growers is making sure each plant is roughly the same height. The yellowing leaves looks like a nitrogen deficiency issue, which could have any number of causes. It looks like you're growing in miracle grow, so I would guess it's over-fertilization due to using too much water when watering, which is really hard not to do with MG.
So my advice would be to water in less amounts, stop using nutrients if you are because they're already in your soil, and move the light a lot closer. Just having the light closer will help your plants with their overall healthyness, I would wager.