First Time Hydroo grow, looking for any advice.

This is my first grow and I figured I'd go right into hydroponics. Ive done a pretty good amount of research so I do have some knowledge of the whole process. I am just looking to see if anyone can see anything I am doing wrong, or could be doing different.

My 2 Plants (AK48 and Northern Lights) are in GH hydrofarms with the drip ring and I installed air stones to it as well, so it is basically a DWC grow.
400 MH bulb with an air cooled ballast, hung about 2.5 feet over the plants. Runs a 20/4 cycle.
1/2 strength GH Flora Series nutrients
The pH fluctuates but is usually around 6. No higher then 6.5 and no lower that 5.5.
Using tap water which is about 120ppm where I live.
Temperature stays around 75 degrees and humidity is typically in the high forty/lower fifty percent area.

Pics are below. Please tell me what you all think. :rasta:

Attachment 294104Attachment 294103Attachment 294102Attachment 294101

There is also some yellowing on the tips of some of the AK48 leaves. Any ideas what that could be? nute burn?