Put new fish in large container/bucket and do an airline tubing drip. Between the tank and container, install a valve in the air tubing. Start a siphon from the tank into the container. Close the valve to the point that the siphon water only drips into the container. A slow drip over several hours will change the water gradually which is important. Be sure the air tubing is secured to the tank rim and have the tubing above the water line in the container so you can see it drip. 6.6 to 8.4 is a large PH difference, so a slow gradual change is imperative. Of course.....keep an eye on it so as to prevent over filling the container.