I have added a florescent light to my trays and thinned out the tomatoes plants to see if that helps. A website I visit said they need up to 16 hours of light a day and that is why they are stringy.
Planted a square of peas, spinach, leaf lettuce, onions, radishes, 2 squares of asparagus and 8 squares of potatoes.
I attached another layer to another box, filled it, and added the string to make the squares and planted onions and radishes.