First grow....Yellow lower leaves ?

I'm brand new to weed growing and literally 2 weeks into my first grow.
I planted my clones 2 weeks ago and have noticed that the lower leaves are slightly yellow while all the leaves above are growing nice and green. There's no drooping on the leaves and I have 4 little babies growing in DWC bubble boxes.
I was using just water for the first week and this second week 1/4 strength nutes while my dimmable balast is running at 250 watts at the moment ? Should I increase the wattage or hold off a little ?
I'm worried about the yellow leaves !! Is this normal please

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A picture would help.You can probably leave it at 250 for a while longer.What's a bubble box? Sounds like you doing good.

Lower leaf yellowing I reckon is a combination of two things--one--your plants roots are not as well developed having justsprouted from clones---two--the plant is not getting enough nitrogen because of the roots and the 250w light is making the plant want to grow fast. I reckon the result is the same as underfertilising, feed a little more and back your light up for a bit--move it further away--allow the roots to form , perhaps water only when the soil has dried out a bit--this makes the roots grow faster in search of water

Thanks to the both of you for the speedy replies.
The clones were actually tiny when I planted them so I guess it's possibly a little early for me to worry yet.
I'm growing in clay pebbles by the way and using house and garden nutes !!
The bubble box is a DWC unit built by nutriculture and combines the upper drip ring and lower resevoir with airstones that fit's 2 lil babies in each (I've got 2 units).
Thanks again

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your cotyledons are dying, its a nature of the plant, don't worry. they'll just fall off and the plant will grow naturally.