Symptoms suggest magnesium as others have pointed out. What makes this a bit harder to tell is they were at a much higher light intensity not too long ago. Too much light intensity will burn leaf tips and aggravate nutrient burn/nutrient deficiencies. Since you've already dialed back the intensity, watch the new growth. The old growth will not heal. Once you have healthy new growth, you can be confident you're heading in the right direction. Several farmers already have you pointing in the right direction on resolving things.
By the way, Chem Dog is kind of a finicky strain ... at least Greenhouse Seed's version is. I took clones and my clones are looking much better than the plants I grew from seed. For the clones, I've been using GH Flora series