Nute burn, heat stress, or something else?

[VladTheInhaler]

This is Thai stick (thai haze x skunk, or so I'm told) clone in its 7th week of flower under hps. I grow in soil mixture withn quartz sand used for draining (store bought mix), in a dr bud- type of micro SOG setup. I feed it every 3 -4 days with 6-8-8 fertilizer which is supposed to have seaweed extract in it, I use plain tap water for the solution. I'm trying to keep the temps in 20 -23 degrees C, don't know about the humidity levels, I don't have a hygrometer.
This is the oldest plant that I have. It was doing okay up till about 2 weeks ago. Here are pics of the leaves:

What am I doing wrong, except the fact that I'm growing a sativa dominant strain that grows 6 ft tall in a 2,5 ft box?

 

[Secret Squirrel]

What is your PPM you are feeding it?What is the run off PPM ?

Do you know your PH?

and allso Sativas take less nutes then indica's.

And if you in soil I would try the nute's one watering and then the next 2 waters just add plane water.try that cycle.

Allso Have you sprayed any thing on your plants lately or while the lights were on?

And does the whole plant look like that or only those leaves in the photos?

Some more info and photos would help.

When in doubt flush them out.
Sir.S.S.

 

[VladTheInhaler]

The top and bottom leaves are unaffected. This is sort of in the middle.
I'm not misting the plants.

 

[VladTheInhaler]

Ok, a little update - same thing started to happen to a 2 weeks younger clone, this seems to affect only the leaves that are in the middle, the top and bottom fan leaves seem to be unaffected. It starts with the tips, which start turning dark yellow in the tips and then it spreads to the edges, like in the picture. The leaf tips point upwards.

 

[guest1ab]

maybe severe light burn?

 

[Secret Squirrel]

I think we would be able to help better If we had a photo of the whole plant not just a leaf.

Sir.S.S.

 

[VladTheInhaler]

Now that I look more at it, I think it might be P def. Here's the picture of the whole plant:

Coudln't upload a bigger pic, it kept saying that the image was too large.

 

[HindukushMaster]

light burn it looks like to me, those things look fried somewhat. What ph is your water/ nute water when watering.

 

[VladTheInhaler]

Water pH is 6,5 - 6,8.
Don't know about light bleaching though, the top of the plant is almost 10 inches from the light (150w hps, cooltubed) and the pattern of light burns would be more random, I think, not like this - starting at the tip and then spreading along the edges.