I want to start my first marine tank - what are the fish to avoid?

Unfortunately, Damselfishes are a problem in new setups and some species such as Neoglyphidodon or Dascyllus are only really suitable if you've got an otter or a pelican to feed! The days of adding hardy fish to mature a tank are long gone thanks to mature live rock that can be purchased ready-to-go, and tolerance of ammonia or nitrite shouldn't be a factor when selecting fish. The time to add Damsels is later on, when less territorial species are established and new fish have to fight their way in.

Other fish that can prove too territorial in the early stages are Dottybacks (Pictochromis) and some Hawkfishes (Cirrhitichthys). Triggerfishes (Rhineacanthus et al) are amazing - intelligent, colourful but often possessed of a mean streak that makes keeping them a challenge. Aside from territorial aggression there's a host of other reasons for avoiding a number of fishes that range from adult size to dietary specialisations. The internet is a wonderful resource but buying a good book is always recommended to help separate fact from opinion. Those by Scott W. Michael are particularly good and in many cases cheaper than a single fish. They'll also save you a fortune in wrong, ill-informed decisions.