Earthbox tomatoes looking tired...

[Tomatovator]

I know you are not supposed to add any ferts to these planters once they are set up but my plants are looking tired. There are plenty of tomatoes set but I have not seen much new leaf growth in a few weeks. Can I add anything to the water to encourage some new growth and bring back that deep green color they had in the spring. They are very pale and need something. I think.