What's been said about water is wrong. You don't pH water because in doing so, you've altered its content which isn't necessary and can harmful. Soil pH isn't consistant from one area to another. pH'ing water implies that 1) soil pH is the same throughout, and 2) that changing pH of water puts the soil in desirable pH range from top to bottom. It doesn't. pH varies in soil to such a degree that it can't be manipulated without reducing effectiveness. The pH charts that people keep putting up here are worthless for that reason. The chart implies that good pH is in a zone that prompts nutrient uptake across the board, meaning all nutrients can be absorbed properly provided that soil pH sits in a zone of, say, 6.2-6.5 for example. That idea is only partially correct... and misleading as it prompts people to think they can put soil in that optimal zone by pH'ing their water. This idea is wrong. The idea that microbes reach a point where they become inactive it also wrong. The only way that would happen is if they run completely out of food. Pretty unlikely. That's the value of a supersoil mix as the granular ferts provide tons of food for microbes.
Flushing is another silly issue that people believe. On the surface flushing makes sense if you believe that something is wrong with the soil, however, the best and only real way to grow in soil in to get it right at the beginning and avoid controlling the contents as the more you do that, the more you screw it up. FFOF is designed to give a grower the freedom to just add water. Your supposed to start plants in FF's light warrior or similar soil designed for seedlings, let the plant get to about 1 foot tall, then transplant into FFOF. Just add water. Don't pH it.
The most common thing I see in this forum is people complaining of problems and they always have details that indicate they are manipulating the grow by doing things to the soil - flushing, adding ferts, ph'ing. If you're using FFOF you don't really need to do that stuff. You might get to the final weeks and discover that you have some early leaf yellowing. If so, it might be a good idea to give a little feeding. However, even this can be avoided if you simply buy more FFOF soil and bigger container. So I guess you have a choice. You can either buy more soil or you can buy bottled ferts. One will give you a nearly guaranteed good grow, and the other will ruin it.
One of the hard parts of growing on your own is walking into a grow shop. They have all kinds of great-looking stuff and people develop that idea that they need at least some of it to have a great grow. Surprise! All you really need is good soil and un-pH'ed water. Pretty simple.
scroll up to the 48:00 minute mark where she talks about pH.