Help whats wrong with my plants

Hey all;

im a first time grower using a hydro set up. Not too sure what to think about my 3 plants at the moment.
Their (first) bottom leaves are a little dark, twisted, and one plant has a crack and a brown spot. They are currently under a 1000 watt MH light, about 2 feet away. Temp(lights on 78, lights off 66) and humidity(40-50%) good.

Currently they are about 3-4 inches tall, and on 1/2 strength nutes being watered 2-3 times a day. I had scheduled to move them to full strength as of monday night. Roots of two plants have now reached the reservoir. Gradually introducing them to the nutes.

I have attached photos.

Please help. Not sure if i should be worried or if i am worrying for nothing.
The strain is Bubblegum, seeds bought from marijuana-seeds.nl

Thanks

oh yeah my res temp is about 72 deg, and my ph is around 5.5 to 6.0.( i have shitty cheap droplet ph tester)
I also have a fan on them currently 24/7 but i have since changed it to come on when the lights come on.

Any help appreciated.
Cheers.

I was told it could be that i gave them some nutes to early and possibly a little over watering. The new growth looks good.

Can any one else agree or give me another reason.

Should i worry about this?

Thanks