From what I've read over the last 10 months or so about Prime (which I use), one figures the amount of Prime needed by the size of the tank to which the primed water will be added. For example, I set aside (Brita) filtered water in 3L- to 1-gallon-size containers (that used to hold distilled or spring water bought at stores). I add water up to about 3/4 of the container, then add 6-8 drops of prime and then top off the container, figuring that the last bit of water helps the Prime mix in, though the Prime could go first if you want. Then I set the containers aside, usually atop the warm refrigerator, to await the next time I do a water change. I also keep water ready in case of an emergency (none of those yet fortunately). So, to review, if you have, say, a ten-gallon tank and a one-gallon container for stored water, you would want to add the recommended dose of prime for a ten-gallon tank to the one-gallon set-aside container.
