The tarp method is the most efficient way of exchanging the water without emptying the pool. Just spread a plastic tarp over the pool, with minimum overlap equal to the height of the pool sides. Weight the edges, I used some firewood I had handy. Using sections of hose and a water transfer pump available at any big box store, pump the old pool water out from under the tarp. At the same time, add new tap water on top of the tarp. When all the old water is out and the tarp is flat on the bottom of the pool, just pull it out.
I tested my tap water to ensure it was iron free. There was no source of iron except the impurities which may have been in the salt I was using. Aquasalt was touted as being the cleanest salt available. I had always resisted using it because it costs about 4x what other brands cost. Since I’ve been using it though, no more iron staining issues.
I’ll add a note of caution regarding the ‘tarp method’ before someone else does. A tarp in the pool could be dangerous if a person or animal were to fall in. Closely monitor the pool while the water transfer is in process.