The ziplock bag won't release any toxic chemicals.
The method I use is to gradually mix DT water into the QT during quarantine so that the water in both tanks is pretty much the same. If you haven't done this, then you need to acclimate the new fish to the DT water as others have said. I imagine your DT and QT temperatures are similar.
To reduce stress during transfer, I use a plastic container like they have at the lfs and herd the fish into the plastic container. I then transfer the fish into another plastic container with DT water in it to rinse the fish off and then transfer the fish to the DT. I use my hands for the transfer (other than on a fish like a lionfish or a trigger) rather than a net. Try to keep the fish calm (back off if you have to chase it to catch it), keep the lights in the room low, and make the transfer into the DT around near the time your tank lights turn off for the night. If there is any aggression, I turn the tank and room lights out immediately.
As Albano says, if your DT has aggressive fish, you will do the newcomer a favor by putting it in an acclimation box for a couple of days so the other fish get used to its presence without being able to get to the newcomer. The acclimation box can be 12" x 12" sheets of plastic eggcrate (lighting diffuser from Home Depot) held together with wire ties.
