Leafs dying off yellow/dark brown/ black

You have a classic phosphorus deficiency working here... classic! And yes, it is often accompanied by a calcium deficiency and yes, pH MUST be monitored and maintained at all times - and is most likely to have caused this... you should flush your plants with 3x the pot volume of pH'd water (for you that would mean running 45L of pH'd water through each pot)... make sure the pH of all water you run through is between 6.0 and 6.5 (and do this with your nute solution, too, AFTER you add the nutes)... Into the last bit of water, add 1/2 strength nutes adding in a 1/2 dose of calmag - but you've got to also cut the Bio Grow down even further - cut IT to 1/4 strength... Now let them dry out completely before giving them a drink again. When you next feed the plant, take all but the Bio Grow back up to the strength you had them... keep the Bio Grow at half of what you had been giving. You might also want to add a good PK booster to your nutrient mix. Plants, as they come into flower, need far more P and K and less of the N that had been given in the veg state. PK works on the buds, fattening them up and this is what the plant is most concentrated on doing. Don't EVER not pay attention to the pH... ignoring that will cause many, many problems.... get yourself a good pH pen - they're worth their weight in gold when it comes to growing weed. Good luck... Oh - if you want a good article to read about phosphorus deficiency, I recommend this one (the very first photo here looks like YOUR photo was used! LOL!) https://www.growweedeasy.com/cannabis-plant-problems/phosphorus-deficiency