Thanks for your help guys, the plants are looking good now, it looks like it was the lack of nutes causing the yellowing of the leaves as you can see from the pics they are starting to look nice and healthy.

We've lowered the lights slightly and the fan is back on full pelt. By the way, we are using a metal halide 250w not a cfl like I said in one of my other posts.

Is it right that they can quadruple in size once you start flowering? I have planted four 'Ice' seeds and eight 'bubbleicious'.