Some just refill and empty the tank several times, but with 100 g. per shot at my local water rate, one is plenty, and in the past even multiple flushes left enough chlorine for the Mrs. to pick up on. Dissolving one or more cups of soda in warm water to the extent possible, then adding via a hose or the already-in-place injector with a tank, or even less, of water, running all the faucets, letting it set a few hours to let the soda absorb the chlorine, and emptying works for me.
I've read where some use a cup or two of vinegar instead, but haven't tried it. I might, because getting the soda to dissolve, and not clump inside the injector, can be a pain. Maybe vinegar plus soda because the vinegar, an acid, would help dissolve and keep in solution the soda, a base. Of course together they'd foam in the injector, so I might try a cup of soda first through it with say 25 gallons, then a cup of vinegar - white, not Apple cider vinegar - with another 25; that way the vinegar might help remove stubborn soda from the injector bowl. An experiment I may try next time.
Joel