so what your saying is that when a plant is in veg, it needs ABC nutes, and when its in flower, it needs XYZ nutes, and that the day the lights change from 20/4 to 12/12 the nutrients needed change instantly?
im sorry but that sounds retarded. when you change the lighting, the plant doesnt make buds that day, it takes about 2 weeks to really sex the plant, and another week or 2 for the buds to start developing. during that month of transitioning, the plant reduces the veg nutes needed, and increases the flower nutes needed.
in my point of view, your stressing your plant by overwatering/drowning it, and your starving your plant of veg nutes right as it decides to cut down on them.
i feed veg and bloom nutes throughout the entire grow, and i flush never. the only thing i change, other than the lighting, is the RATIO of nutes. many people say they have nitrogen deficiencies during flower, well yea, if your flushing out the N, thats gonna happen. the plant still needs veg nutes during flowering, simply because theres still growth of stems, and fan leaves, and sugar leaves... also, potassium and phosphorus dont only work in flower, they help rooting during vegetative growth, and many other things.
12 gallons of water? wtf? try what im saying, for 1 plant, and after a few months, id be surprised if you didnt see a difference. just start cutting down on veg nutes about 2 weeks before you switch them, and start upping the amount of bloom nutes.
I just cannot agree that flushing a plant the day you switch it to flowering is healthy, and hell, its a waste of water, and nutrients.