Did you add veg nutes? If so, how much, how often? If not, it's way past due for some nitrogen.

If it's getting that bad, perhaps a gentle transplant into fresh soil? ('tapping off' the loose stuff before replant) If the added calcium is the problem, flushing wouldn't solve it.

A ph pen and calibration solution would be preferable. Lasts longer. Knowing the difference in ph of in-going water and run-off water can help with troubleshooting for virtually every grow.