Raising and lowering the pH of your growing medium is extremely important throughout the entire life of the plant. If your pH is too high or too low, your plant will not grow as it should.
Neil from Got Pot Cheaply recently made a video about his garden that has autoflower plants of Think Different strain. He is using a 600w Apollo HPS bulb setup and his tallest plant is 18". His cannabis plants are starting to pre-flower and he expects them to double in size. He has been watering every 2 days vs every day and he has also been tucking the leaves under the buds so the buds get more light.
In the video below he also talks about raising and lowering the pH. He uses the below products to ensure not only his pH is correct, but his PPM as well.
Check out his video here: