And what are you basing what you are saying on An opinion and nothing more? If trace metals are what you want to remove from your water the only sure fire thing to remove all the trace metals etc. from water is a reverse osmosis water filter. Putting chemicals into water to remove elements is just adding more crap into the water for you turtle to "allegedly" aborb into it's skin.
On another note in all my years of doing rehab on turtles and running a rescue I can honestly say that I have seen it all and let me tell you. RED EARED SLIDER CAN SURVIVE IN SOME PRETTY HORRID CONDITIONS. I have rescued them from living in years of pure sewage filthy waste water that they were swimming around in like it was nothing. If you drank that water you would find yourself in the hospital in a matter minutes with salmonella, Giardia, and God knows what else. So how does your "theory" of absorption ratios really work here?
Frankly -- I would listen to a sucessful hobbiest's husbandry practices before I would listen to someone who just has an opinion that does not seem to be based on anything factual.
This person states that human safe water is safe for turtles. That having been said by someone with 25 years of successful keeping holds much more weight than what you are saying.
Sure unhealthy water that is not safe for human consumption is not the ideal situation for the animal and should be avoided or run through and RO filter, but no one is arguing that point with you. All that is being said is that human safe water is turtle water and that is a fact not an opinion plain and simple.