Many people try to avoid cautionary notes and go for the animal they want because they are attractive, interesting or colorful or just easy to keep. Although many people have anecdotal success with these creatures, the preponderance of posts/experiences from long time reefers are negative ( at least in smaller tanks). Some of the animals that people like to add but cause headaches are:
1) Coral banded shrimp (CBS)
2) Sally lightfoots
3) Sea Stars
4) Cucumbers, sea apples, nudibranchs
5) Hawkfish
6) Yellow colony polyps, xenia, large stinging corals such as galaxea
7) Anemones in small tanks or under inadequate lighting
8) Arrow crabs
9) Certain Damselfish and dottybacks
To name a few........SH