2 weeks and theyre 20cm tall? How far away is the light?
Edit: get the 400w instead, Im growing 4 plants under a 600w HPS in a closet - 2.5ft wide, 2.5ft deep and 5 feet high, and Im only just able to control the temps. And dont put a sheet of glass under the light to try and make it stronger - it doesnt. Glass absorbs light.