Don't dump bag water into tank - why?

Probably due to the dubious quality of water from the pet store..

I personally don't worry about moving water between my own tanks.. they're all pretty much the same parameter wise.

If you get fish from the store the bag water obviously has their water in it. Pet Store Water isn't the cleanest thing usually. If you dump the water in you let anything living in that water in your tank. Which can be much more of a hassle than catching the fish. I personally get a 5 gallon bucket, have my dad hold the net over it, and dump the bag into the net, water going into the bucket. Dump the water down the sink of whatever. Good Luck with your fish.

If you have multiple tanks known to be disease free for a long period of time then mixing the water from the tanks is not a problem. Pet store water is a different story. The pet stores keep getting restocked by wholesalers and don't always have the space/time to QT every fish to guarantee a disease free tank. While your fish might not be infected, the water in the petstore bag could carry a whole range of nasty stuff that can enter your tank water and cause a whole bunch of problems. If not a disease you could bring home black beard algae, red algae, beard algae, cyanobacteria ect. that will cause you tons of grief which a simple 1 minute task of netting the fish could have saved you from.

Not to mention parasites that may not necessarily be on the fish, but it the water!