Big mistakes made: Water Change Questions

It would take a fairly extended period of time for it to be an issue. The directions for Prime state if re-filling to the tank directly, dose for the volume of the tank vice the amount you are adding back.
 
Is it preferred to add the Prime to the change water you're adding, or just add it to the whole tank right after you do a water change?

I have been adding it to the buckets of water that I am using to do water changes with, then adding that water into the tank.

i treat my whole tank with prime then toss the water in.
 
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