What’s wrong with this plant

I added more coco to the pot now it’s standing better than ever, still not fully recovered yet

To me, this looks like environmental stress. So it could be overwatering or an issue with temperatures being too high or too low. How confident are you of those thermometer readings? Were they made at canopy level directly under the light? Is that the peak daytime temperature? What's the lowest temperature you get at night? 600W HPS kick out a lot of heat, let alone 1,600W, so that's why I'm suspecting this first. If you're absolutely adamant it's not the air in the tent, then it must be something in the roots. What temperature is the water you're feeding them? If it's below 16°C then this might be the issue. Overfertilization could potentially cause curling but I'd expect some browning leaf tips and crispy bits too after a while, and it usually causes pistils to brown and shrink earlier than usual as well.

Hmmm, if everything in the tent has been treated the same and this one in particular is different then it could be. Genetic: This strain may prefer lower dose nutes and less light intensity than others. Have a more delicate root system....... Environment : air flow reduces as the tent fills during veg and flower, this can cause dead spots where air doesn't move. (add an oscillating fan). It is very rarely something wrong with the plant. We cultivators don't grow anything. Providing the ideal environment is the name of the game.

Ok first all my plants had too much nitrogen then I was watering with just water until they looked better, this one didn’t end up looking too good after that saga. I water with 6.5ph always Temp 26 degrees Celsius Humidity 40-50 18 hours light 1000w hps and 600w hps

I've always found purple strains more of a challenge. I find them sensitive to ph changes, and high light intensity. Without more details I can only guess. Are your environmental controls dialed in?