Why my plants have yellow tips

Way, way over fed and sufering from nitrogen toxicity.

Heavily overfed. Slow down .

500 ppfd over 18h is fine for sure.. without hours of operation can't assume the DLI. That's 47.5 DLI over 24 horus and that would be too much. 18 hours is 35.6, which is most likely fine, but some envrionments may even have a problem with that if VPD is too high. sounds like you have bad watering habits based on your wants rather than the plant's. You wait until it dries back to a consistent point and repeat (this is where your preferences can deviate a bit as long as not too quick to re-irrigate). Don't over-complicated it. If it takes 2-3 days, that's just what it takes. It's not foie gras. Also, If simply watering causes droop, that's a self-inflicted problem. you need more aeration/drainage amendments in your substrate next time. coco only needs 33% (2:1 ratio) or higher water capacity soils need 50% (1:1 ratio with perlite)... and i'd actually recommend vermiculite #3 over perlite. holds some water and provides available silica. Irrigating properly should never cause droop. that is cuased by something completely avoidable. those are not Mg symptoms at the top. Damage starting around the tips and margins are most often K-deficiency related. The plant is also fairly dark, so it's probably been overfed a bit. It's possible ther is plenty of K present, but something else is inhibiting its availability. chlorosis in new growth is related to too much p or k, which makes me think this is more of an issue with ratios of nutes provided over time more so than lack of a physical presence. interveinal chlorosis on lower leaves may or may not be related. more than one thing can cause that like most other symptoms, so you need more information to confidently diagnose. if it gets some spots along with the interveinal chlorosis, that would be mg-deficiency symptoms, but without spots, it's something else. Symptoms are indiscrete, if you don't keep track of how you fed and what ratio of nutes over time, you are just stabbing blindly in the dark... or at least blind in 1 eye and an arm tied behind your back. Whatever you choose to do, try one thing at a time so as not to confuse cause and effect.

Turn down your lights a bit and add 1 teaspoon of epsom salts per gallon of water. Those plants show no sign of calcium issues, only magnesium. Using calmag when there's no sign of calcium deficiencies can lead to nutrient lock-out later in the grow. Excess calcium can lock out pretty much everything else. There's also not much calcium being used during veg. Once the plant is flowering, calcium needs go through the roof. That would be when to add calmag. Until then, epsom salts should be enough without leading to lock-out. Once again, turn down the light. It's what is causing the magnesium issue in the first place .... you're trying to push your plants too hard.

Magnesium? Have you been adding magnesium? Calmag perhaps? If not then probably not magnesium. Either way flush and maybe let the plant tell you more. If it’s excess nutrients you can not fertilize for a week and let the plant use up the extra nutrients. I don’t think you are dealing a deficiency problem. They are just t to healthy otherwise. Last thought, if you’re transitioning into flower and you have not yet started calmag that would explain things also. Happy gardening

My guess would be too much Nitrogen. All the other foliage is super luscious which is a indication of abundance of nitrogen. Flush with ph’d ballanced water and then back to recommended watering schedule. Happy gardening

potassium and magnesium by the visual looks of it but that doesnt make much sense atleast to me. i would check your ph. everything else looks good. like theres good nitrogen availability. maybe its too far on one end of the scale and limiting phosphorous and