What are these on my leaves?

Calcium has been shown to be best absorbed at a pH of 6.2. Your plants certainly need to be flushed because i see several issues, including nutrient burn. I would guess your soil pH isn't in a acceptable range and you have salt build up. Flush your soil with pH 6.2 adjusted water and make sure there is a lot of run off out the bottom. You wanna wash out all that salt that's been building up over time. After you've done that, then you allow for the soil to dry out till your pot feels light. Then re-feed and add a Cal-mag additive. And always make sure to pH your nutrient solution to 6.2

Start slowly adjusting up to 6.5 with every watering. No need to do anything drastic. For next grow best to keep in mind that anything you use to lower PH can also build up in your soil by time. Your plants will make you happy soon! 😁

I was told ph is best at 6.2 as its slightly acidic but below 7 and above 5. should it be lower?