Mate, that is seriously advanced Mg defc. happening, for some time by the looks of it,... the flushing made matters worse, so don't do that anymore unless there's a dire need to purge the medium of excess nutes or it badly off pH,...
Biobizz is very mild, and reducing the feeds dosage will start starving the plants slowly, triggering defc.'s like this,.... start feeding full strength, and start your Ca-Mg supplement right away! If you can, do foliar sprays as well to get caught up faster; foliar is the fastest way into the plant,... you'll need a sprayer, a Ca-Mg supp' (or epsoms, which are MgSO4, used at 1/2tsp/L), and a wetting agent, or a mild soap like (no fragrance) castille soap.... these help the solution coat and stick better, rather than just bead up; get tops
and bottoms of the leaves... More stomata are on the undersides vs. the tops,... do this away from direct lighting until dry, and make sure she's dry by lights out,.... 2-3 rounds will be best... Mg defc. symptoms can be reversed some, depending on how hammered the leaves are, past a point they won't correct....
Water doesn't have significant Mg in it, only CaCO3 if it got some hardness to it,.. even then, that doesn't mean the Ca is free and available for uptake,...