Superthrive is great for helping build a strong root system, three drops per gallon of water. Also unsulphered molasses - at one tbsp per gallon, once a month, helps feed the positive critters in your soil. Both pretty cheap, too, and can be found at Walmart. (The molasses will be in your cereal isle, by the syrup - usually Grandma's).
EDIT: Sorry, I'm pretty sick - got pneumonia - you don't grow in soil, do you? I don't know if there would be a benefit to adding the molasses to a hydro or aero setup - somthing you might wanna look into, though. Also - a bit of hydrogen peroxide in those systems can keep bad bacteria under control, I can't say how much though.