Pre Flowering Flush

I keep reading that a soil flush, equal to water 3X the amount of soil should be used before going from vegetative to flowering cycle.
Thus for a 3 gallon container 9 gallons of water?

Do I really need to use that much water, and I am guessing it must be distilled or purified water at that!

Any advice?

Just my opinon I think that flush is for dummies who over nute and have to quick flush before harvest.Just water with the cleanest water you can for the last 14-21 days and you will have the cleanest smoke with the whitest ash.

I've never heard of flushing between veg and flower. It seems like a bad idea too, since the plants still need veg nutes for the first couple of weeks of the flower phase.

its good to occasionally flush to prevent salt buildups. the idea of flushing between the two would be that the plant needs flowering nutes instead of veg when you start flowering im going to assume. however the plant actually increases nitrogen usage drastically during the stretch phase of flowering when they will around triple in size. I use flowering nutes around once every 2 or 3 veg feedings depending on how close the stretch is to being over. so I would not follow that advice of flushing between, the whole idea has very large cracks in it. where do you keep reading it? i hope it wasnt from this site.

I've personally never had to flush. I've held back nutes a bit when I was afraid things were getting too hot. I follow General hydroponics instructions and try to run through about 20% of the water through the drains with my feedings. Then every 3rd water I do the plain stuff and let that run through a bit. If I'm lazy or short on time I'll sometimes under water a bit so there's no clean up, but it's probably not healthy.

In regards to water type, my tap is 80-130 ppm which ppl would probably consider acceptable to use. I got a reverse osmosis system anyway, and it made a huge difference. For example it basically doubled the speed my aero system roots. Tap will probably be fine if ur waternquality is decent Though. Even at my lower PPM my water would leave buildup where ever I sprayed it, wich prob isn't good.

How much water Really depends on your medium. In a high porosity soiless, augmented with extra perlite, about 4-5 gallons of water is about perfect for 15 dry gallons for me. I get a little run through. I put only about .5 to 1.5 gallon in my 5 gallon pots.