Anyone Know what this is? Mg? burn?

I have a small grow going of 4 plants with only one plant developing brown spots starting in the older leaves and working its way up tarting at the leaf tip. I have attached some pics.

Its a DWC grow using the lucas method.
600ppm @ 5.7-5.8ppm
Strain = Critical Jack

Any help would be appreciated.

Thanks

-Sausage

 

Thanks for the advice. Ill ph up to 6.0 and see if it likes that.