Brown Marks, Drooping leaves and pH falling down

Hi mate, My last grow was RQS NL, I had the exact same problem as you have, unfortunately I never figured it out, it got worse but still got 2 and half oz from her, i was left assuming it was a root issue even though they looked fine, I had other strains going in same tent which didn't get this problem as bad but they still had a crashing PH. I was using tap water, bubbled for 24 hrs to remove chlorine, however it doesn't remove the chlormine, in the end I put the issue down to a build up of chlormine in the roots, so for my current grow I bought ecothrive neutralise and plant success orca, my PH is now stable, only raising .3 as the nutes get used, and only crashed once due to low ec. Not sure if my chlormine theory is correct, but I'm not having problems with this grow, I would try the neutralise though as chlorine and chlormine are not great for plants, or beneficial bacteria like hydroguard. I would also raise the PH to 6 or 6.2 calcium is absorbed better above 6. I'm a newish grower so could be totally wrong, just trying to offer some help. If your using RO water ignore everything I said lol :)