You will be amazed at how things get into your tank that you did not want there. Even inverts can bring them in, and many spores can remain dormant for a long period of time before conditions are favorable then spring into life. Unfortunately you cannot dip for everything and the only advice I would want to give you is if you strive to keep stuff out of your tank, do not ever put a frag plug into your tank. It is amazing how much stuff can hide and survive on a frag plug. Remove all corals from their plugs.