I always dose my tanks with the full volume of water it holds. That's what is on the directions. If unable to treat ONLY the new water before putting it into the tank, then to dose for the full amount of water.
I realize this may just be for the purpose of making more money, but I buy Prime in bulk, and it lasts me quite a long time. I feel the extra money is worth the peace of mind when it comes to the many many tanks I have that are very large and have water changes of a LOT of tanks with a LOT of fish that I love like my children.
Maybe I'm too cautious, but that's what I do, and everyone I know does it here in the aquarium club. You never know if your municipality starts having issues, and the water parameters change...they only have to put out the statistics once a year. We had a huge pH change when we got a lot of rain, or something. All of our water originates in the man-made lakes around here. Sometimes they change the amount of chlorine. Sometimes my water doesn't smell at all, but a month later, it'll smell very strong of chlorine. We have a small RO that we get our drinking water from.
