Top Leaves yellowing

Looks like light burn to me .

By look of your pic first thoughts are what’s the Ph run off after 15%..what’s the nutrient schedule..possible Ph and nutrient burn..flush with 5.8-6.3 Ph until run off is in range...once dry 1/2 recommendations on nutrients...also check light distance as they veg and grow into light..

Sidenote: according to the ppfd map for the Lumatek ATS300 (I believe this is the one you have), you should be 35-40cm above the canopy during flower. This would give your plants 700-900 micromoles.

Looks like you're 2-3 weeks into flower. I've found it's often at this point where it's need for nitrogen drops to almost nothing. The symptoms also fit. Dark foliage (although you can have toxicity in flower without the dark green) and twisted yellow tips on the tops. I would pull the N back as far as possible (0 if possible) for a week or so. This will give her a chance to use up any residual N left in the soil. Reintroduce your N after but still keep it to a minimum.

can it maybe also be the light to intesnive? i have a Lumatek Attis running at 300w at around 45-50cm/17"-20" above the canopy