Before going out and purchasing additives:
Keep in mind that the effective (usable) light from your 400w bulb only extends about 3 feet maximum. Including the distance from the light to the tops of your plants, should leave you with about 18-24 inches of effective light. (the botom 12 inches of a 3 foot plant won't be getting enough light) No additives will correct this situation.

I would however recommend that you start giving a source of calcium pretty soon. I recommend 1 teaspoon per gallon of unsulphered molasses per gallon of properly ph'd water...once a week. Molasses is a cheap and very effective replacement for stuff like CalMag Plus, Sweet, most Carbo-Load products.