Of course prime wants you to treat the entire tank volume..... They make more $$$$.
Actually, this isn't a case of Seachem attempting to bamboozle people into using more of their product to increase their revenue (if it was they wouldn't have created a concentrated dry powder product such as Safe), it's a case of CYA (cover your ass), so that when some numb nuts wipes out 50K worth of coral &/or fish because they didn't understand their local water conditions, or can't read a label, they can't blame Seachem. This is what happens when you run a large corporation, based in the land of lawsuits.
As Dr. Greg Morin explained in the link that I supplied in my initial comment;
"some people prefer to just add the new untreated water directly to the tank... if they do that then we recommend the amount of Prime they add be based on the total gallonage of the aquarium rather than just what they added. The "extra" amount speeds up the rate of removal."
But, at the end of the day, if what you are doing works and does not cause any problems then it is ok. Our recommendations are meant to cover a broad range of users and we tend to prefer to err on the side of being overly cautious.
Which goes directly back to exactly what I have been saying, there is no single definitive way to treat water that will fit every situation encountered by every member here on MFK.
For "some" people treating the entire tank volume (even if adding dechlor in stages) makes perfect sense, and is not a waste, but an added layer of protection for their livestock.
Some hobbyists keep very sensitive species of fish, some keep freshwater, some keep SW and the various sensitive corals & invertebrates that live within a marine system, some have heavily planted tanks, some have no plants, some keep their tank water at 65F, some keep their tanks at 88F, some have pH values as low as 4.5, some have pH values as high as 9.0, some have tap water treated with chlorine, some have water treated with chloramine etc-etc- and all of these various paramaters and values can & will affect the reaction time and toxicity of disinfectants such as chlorine and chloramine. The VAST MAJORITY of hobbyists (including here on MFK) don't even know what the level of disinfectant residuals are in their tap water as it leaves their taps.
Who here has actually taken the time to test their tap water for chlorine/chloramine levels, and for those that have what did it test out at, and is it that rate consistently year round, or does it vary within the seasons? Do you know the reaction time for the amount of water conditioner that you use within your system, based on the water volume that you change? I do, because I made it my business to know these things, and not rely on someone on MFK to lead me to the promised land.
From what I've seen over the years most people on MFK don't have the slightest clue, many don't even know if their water contains chlorine, or chloramine. They read their water conditioner label & then treat 50 gallons of water with one capful, because that's what it says on their bottle of Prime, when in reality that dosage rate is based on very specific values, and should only be used as a guideline depending on what type of disinfectant ones local water is treated with, and at what level. (mg/l)
Don't blame Seachem for not being mind readers, or for covering their arses with some legalese in a time when people are always wanting to blame someone else for their own ignorance and/or laziness.
It's for this exact reason that I posted the following info;
http://www.monsterfishkeepers.com/forums/showthread.php?309623-Cost-Effective-Water-Conditioners
It probably should have been made a sticky .......
so even if they have to put hat on the label they still make more profit.