i would flush the pot with ph corrected water until the run off is under 6.4
good luck
peace
Ez

By the time they are in flowering at week 6 they look like this...

All the other bud sites look to be blossoming but the very top colas seem to have a white blob...

Do you know what the PH of your soil is? You may have to compensate for an alkaline soil.

What is your lighting, and how far away from the plants is it? Honestly to me it looks like neither Magnesium or Potassium def.

The onset of potassium deficiency is generally characterized by a marginal chlorosis progressing into a dry leathery tan scorch on recently matured leaves. This is followed by increasing interveinal scorching and/or necrosis progressing from the leaf edge to the midrib as the stress increases. As the deficiency progresses, most of the interveinal area becomes necrotic, the veins remain green and the leaves tend to curl and crinkle. Because potassium is very mobile within the plant, symptoms only develop on young leaves in the case of extreme deficiency.

Magnesium def. will also first show an old growth. Both of these typically move from the bottom to top of plant.

It looks more like light burn. But could be possibly a Sulfur deficiency or Zinc deficiency.

Sulfur def. initial symptoms are the yellowing of the entire leaf including veins usually starting with the younger leaves. Leaf tips may yellow and curl downward. Sulfur deficiencies are light green fruit or younger leaves with a lack of succulence. Elongated roots and woody stem and leaf stems turn purple.

Zinc is the most common micronutrient found deficient. First, younger leaves will exhibit inter-veinal chlorosis. New leaves and growing tips develop small thin blades, contort and wrinkle. Leaf tips and later margins discolor and burn. Often confused with a lack of manganese or Fe, but when deficiency is severe, new leaf blades contort and dry out. Also gets locked out due to high pH. Zn, Fe, and Mn deficiencies often occur together, and are usually from a high pH.

I was just goign to say my plants seem to show all of these symptoms then the last sentence summed it up "Also gets locked out due to high pH. Zn, Fe, and Mn deficiencies often occur together, and are usually from a high pH."
My PH is always rising even when I PH water going to 6.0 is there any thing I can do to stop the increasing PH?
By the way Im using a 400W HPS in a cooltube.