I'm another that used to add it directly to the tank before adding water. I'd drain it all with a hose, about 100g from a 125g tank. Then I'd add a double dose of Amquel before refilling. I kept a powerhead near the bottom of the tank so it never had to be shut off. This allowed the new water to be mixed with the old quickly.
Now I said to heck with it all. I bought a 125g nylon cylinder tank and mounted it on a 5' tall stand. I bought a 300g per day RO/DI unit and a float valve. Whenever the cylinder drops 10g the float opens and refills the cylinder with pure RO water every time. I built a small shelf right under the tank itself and plumbed the tank to a small BlueLine external pump with a hose bib on the outlet side. This way I can go up and get a 5g bucket filled quickly just by opening the valve bib and letting gravity do the work or I can add a hose to it and turn the pump on. I can fill 100g in about 10 minutes. Now all my fishies swim in RO/DI whether it's a salt tank or freshwater.