Five gallons isn't necessarily enough water to put into the tank BEFORE you add whatever treatment you use. You need to take a look at how much 3" pipe there is between the tank and the valve and take a guess at how much water that pipe will hold. Dump that much water into the tank, then add a couple more gallons, then add your stuff. If you put your treatment in first, or mix it with that first five gallons or so, all of the treatment will be in that piece of pipe instead of inside of the tank.
Each foot of 3" pipe holds a little over 1/3 of a gallon of water.
Bruce