QT stands for is Quarantine Tank, it's a small tank (usually 10g to 20g) that has no live rock or sand. Paint the bottom outside of the tank black, or place the tank on a towel. Use a small HOB filter, and an air line attached via rubberband to either a piece of PVC or plastic décor. The PVC/decoration is used to let the fish hide and feel secure, the air line attached to it will help to move the surface water and circulate the tank, the bubberband will make sure the air line stays on the bottom of the tank and not float up to the surface.
Quarantine all new critters for at least 4 weeks. If you notice the fish is sick, or has parasites...it's easy to dose the tank since you will know exactly how much water is in it. Also add only one fish at a time...using a QT will help you space how long you wait before adding a new critter, preventing the "too much too soon" tank crash. If you purchased your fish from this site, it will also prevent the CUC (Clean Up Crew of snails, bristle worms and such) from eating the critter, so you can get your 14 day refund if the critter doesn't make it.
Just take a rubber band and attach it to the outside of the PVC. The rubber band is just to keep the airline at the bottom of the tank so it can have a better airation effect