Vinie fan leafs and burnt tips

I'm in need of some help please? I'm in week 6 into veg with all fem Gelat.OG. 2 mothers and 20 clones from the mothers. The room stays at a consistent 78°F and 55-58% humidity. Co2 ppm is set at 1200. I use kingled 1500w led lights set 18"-24" away from the plants. I use FFOF soil and FF nutrients. The nutrients I use twice a week at 1/2 the recommended amount. Lights have been 24/7 since I started this grow. The room is sealed with plastic.

Suggestions please?

First thought I had was it looks like a burn. How are you putting the fluids to them? Saturate the pots fully and then 10 to 20% of the pots volume through it in runoff. Do you water between ferts or just keep putting the 1/2 strength nutes to them. Not being sarcastic or trying to insult you if you've got lots of experience; it's just that it looks like that type of a burn. The Foxfarm soils are hot too. I've seen them burn up autos.
An old vet who worked in a grow shop once told me, "if in doubt; flush them".

Ya im flushing them now. Just straight 6.0ph water now. Before the flush i was watering every 2 days. Every other water has nutrients with it at half recommended chart amount. Im hoping that this water flush will fix it. Its been 4 days and they seem to be coming back around

If you think the salts are really built up in their soil a 1/4 strength nutrient solution is supposed to remove more salts than plain water, but just water is likely going to be good. Hope they do well.