Find out the ph of the fish store you are using. It may be hard to get them to acclimate from 7.0 ppm which is standard neutral....to your 8.0 ppm. You might try a very slow acclimation. I've never had to do this, but other folks here can give you details I'm sure.
Also, I'd stick with the water you've been using for changes.
Also, I'd stick with the water you've been using for changes.