Actually, decant or drain a little of the water in the fish bag, set it in a quart pickle jar and add a little tank water. Watch a half hour TV show (like you've got time!) Decant a little more water. Double the water in the bag with tank water. Do the drain and add one more time, Then dump all of the bag water and add your fish to the tank or quarrantine tank.

Some "bugs" may still come in via the fish's skin or gut, but any disease innoculation from the shop will be much less.

Much of life is playing the odds. Always want to improve the odds for those I care about, even for the critters. :)

Oh and I'm so darn lazy, I use my fingers closed over the jar to drain it. (Don't try that with fry.) It's easier to wash hands than sterilize a net.