A simple tank with live rock, pump and skimmer is set up to house all new corals for inspection and treatment if needed.

Why quarantine?
Because there is an ocean of potential problems you could introduce into your tiny reef ecosystem.
Here is some info on what pests you should look out for and what to do next.

Having reef aquaria since 1994 I have seen / experienced allot

Bad
     Red spot Acropora feeding amphipod, (AKA red bugs, red butt bugs) to the naked eye they look like a red speck or a cluster of red specks on the tip of an Acropora branch. They only affect Acropora species and in my option they are not life threatening to the coral but they do affect its growth and often its color. Do not confuse them with the other larger copepods swimming in your system, these ones are about the size of a hair follicle, or sharp pencil dot. They can swim if disturbed, so simply dipping the coral in an Iodine solution (coral dip) will only kill the mature ones that are holding on and not the eggs or the ones that jumped off. The only way I have rid my system of these pests was by using Interceptor, I found this link The Treatment that describes the procedure. I would also add some new cured live rock to re-establish the natural plankton.

 Very bad if un checked
      Montipora eating nudibranchs are quite a pest if not found early on. They eat the living flesh of  Montipora species and can wipe out all your Montipora corals if left unchecked, commonly first seen on the underside of Montipora capricornis but you must check all species of Montipora for they can be found even on Montipora digitata. They look like white or off-white fluffy spots up to 1/4" long and the best place to look for them is at the base and undersides of the Montipora coral. A tell tale sign of there presence is the appearance of fresh dead areas on the coral. The only sure way to avoid them is to quarantines the new coral for two weeks (incase there are eggs that haven't hatched) and check for there presence. If you find them you should remove them by picking them off or even better use a turkey baster to blast them off (in a separate container), and then scrub the base rock and all dead areas with a tooth brush to remove any possible eggs. If they are already in your system you must be diligent about there removal. You cannot remove them all in just one shot it will take several weeks and even then they may reappear a few months later, so always be on the look out for them.

Very bad and can destroy allot of Acropora very fast
      Acropora eating flat worms (AEFW) Don't confuse these with the common red planaria, unlike the red or rust colored flatworms these worms have no problem climbing onto Acropora branches. The adults are oval in shape, up to 1/2" from front to back and about 1/4" wide. Here they feed on the flesh of the Acropora  coral and the zooxanthellae contained inside. The tell tale signs of these worms are the white holes left behind as they feed. The adult AEFW is very well camouflaged against the surface of the coral and during the daytime they usually hide in the folds between branches. When it comes time to lay eggs the adult AEFW will  eat a section from the base of a colony upon which it will lay its eggs. There are some coral dips that will kill them but not the eggs, but there so far is no in tank treatment like there is for the red bugs. So far the best remedy for removal is the use of small species of Reef-safe wrasses and fallow the method described for removing the Montipora eating nudibranchs.