Join Date: May 2020
Location: Southeastern US
I mean, I agree that the stoichiometry isn't different, but I'm not sure it's a matter of the reaction being fast enough and more physics. Even if we have a small amount of chlorine or chloramine in our tank from the tap water we just added, we want those molecules to come into contact with our water conditioner before they come into contact with our livestock/filter bacteria/etc. - those are the side reactions we are trying to avoid. I assumed that treating the entire tank was to make it much more likely they would be neutralized before doing damage by increasing the water conditioner concentration, even though from a numeric perspective it's an unnecessary amount.
Now, if you want to say that a small amount of chlorine toxicity is not a big deal, well, it comes down the to specific circumstances. I don't worry about drops of tap water in my tank at all, but for larger amounts of tap water I think most people don't have enough information to figure out what is safe and it's way easier just to dose the whole tank.