(Repost with Permission)
Fish to Be Avoided Part II
(Part I) Venomous and/or Toxic Species:
Stonefish (can be deadly)
Lionfish
Rabbitfish/Foxfaces
Scorpionfish
Coral Catfish (these also get up to a foot long and no longer school once larger)
Bluering Octopus (can be deadly)
Toadfish
Canary Blenny (venomous bites that can be painful but little else)
Flower Urchins (can be deadly but rarely encountered in the aquarium trade)
Black Longspine Sea Urchin (can inflict painful wounds, some debate exists whether or not they are really venomous)
Cone Shells (rarely encountered in the aquarium trade, can be deadly)
Stingrays (many have venom associated with the spike on the tail which they use in self defense, don't get stung in the chest and you should live to tell about it)
Sea Snakes (I know of no one attempting to keep them in captivity, but included for good measure)
Box Jellyfish (quite deadly but of no concern to aquarists)
Hell's Fire Anemonen (while all anemones are capable of stinging, this is the one of the few to be concerned about, very painful stings)
Hydroids (usually just cause skin irritation if anything)
Fire Coral (see above)
Zoanthids (some of these can contain Palytoxin which can be quite dangerous and make you very ill, they're quite frequently harmless but if you want to err on the side of caution rubber gloves are a good idea when handling them, as are goggles when fragging them)
Extremely Aggressive Species:
Undulated Triggerfish (the meanest auqarium fish available in all likelyhood)
Queen Triggerfish (not quite as bad as the Undulated, but pretty close and they get very large)
Clown Triggerfish (pretty similar in demeanor to the above two)
Blueline Triggerfish (not so bad when young but a beast once it grows, perhaps the least aggressive of the four)
Passer Angelfish (probably the meanest of all Angelfish, I've seen them take over tanks)
Damselfish (they're not all bad, but ounce for ounce some of them are the meanest fish around, think twice about adding them as some of your first specimens)
Maroon Clownfish (females get quite large and they can get quite mean and bully any tankmates that dare come close, they're also probably the least tolerant of other clown species)
Sohal Tang (hardier than the Clown Tang but just about as mean, probably best to keep them as the lone Tang and if you must keep one in a community reef tank make it your last fish addition)
Inverts To Be Avoided Or Better Left To Experts:
Non-photosynyhetic Corals and Gorgonids [Sun polyps, Carnation, Devils Hand, Chili Coral, etc.] (if it's a soft coral and not green or brown in part and is very vividly colored odds are it's non-photosynthetic and requires more small particles of food than most aquarists are willing to provide, the only non photosynthetic stoney corals frequently seen are Tubastrea sp., regular feedings of meatier foots can lead to success with these)
Christmas Tree Worms (filters feeders that rarely live long in home aquaria)
Coco Worms (see above)
Goniopora sp. (some strides have been made but still miserably low survival rates, stokesi is the most common and seems to be the least hardy in the genus)
Feather Starfish (require huge amounts of flow and large amounts of tiny planktonic organisms)
Basket Starfish
Crown of Thorns Starfish (duh!)
Linkia Starfish (disease issues and poor acclimation to aquarium life, problem feeders as well)
Wild SPS Corals (small-polyped scleractinian) that are not frags (wild colonies can be particularly adapted to flow and light from their natural environment and often do poorly once in aquaria, see out hardy aquacultered specimens)
Sea Apples (often slowly waste away in starve to death if not offered large amounts of food appropriate for filter feeders, also chances of toxins being released and possibly killing other organisms)
Sea Pens (still offered in the aquarium trade but chances of survival are very poor with this filter feeder)
Giant Xenia (this one rarely does well once established and like most other xenia does not ship well)
Sea Slugs and Nudibranchs (very specialized feeders, a couple can be useful to elimate pests but it is very difficult to sustain a food source for even those, they're also very prone to damage by overflows and pumps)
Flame Scallop (filter feeders that usually waste away in home aquaria, the same goes for other Scallops which are les frequently encountered in the trade)
Anemones (most anemones should be placed in specialty tanks and also have very poor survival rates, beginners should not attempt Anemones without extensive research)
Bright Yellow Anemones (dyed; and done most commonly with Sebae, but also seen less frequently with Long Tentacle and Carpet anemones)
Harlequin/Clown Shrimp (must have live feeder starfish to survive)
Camel/Mechanical shrimp (Not reef safe but often sold as as such)
Elegance Coral (recent poor survival possibly due to a disease, other factors might relate to them coming from higher nutrient environments)
Red Serpent Starfish (often disolve and waste away and can be very fragile)
Pipe Organ Coral [Tubipora Musica] (often hacked off from a larger colony, recent survival seems better than in the past)
Large Sponges (often hacked off from large colonies of their rock base, also exposed to air for too long which leads to their demose, bright orange and yellow colors are common)
Special Notes:
Clownfish [Amphiprion sp.] (various species often acclimate poorly to aquarium life and suffer greatly from collection stress, I've seen estimates that as little as five percent of those collected live to be in home aquaria, when possible buy tank raised specimens)
Bangaii/Borneo Cardinals [Pterapogon kauderrni] (rather limited in range and rumors of an unsustainable population if the current rate of collection continues, there are also stories of poor survival after collection, buy tank raised when possible)
Tangs (should have larger aquarium to provide them plenty of swimming room, no a tang is not suitable for your nano cube or 29 gallon tank, when small 3'-4' aquariums can be suitable for short periods of time, though bigger is recommended by many, just be sure you're planning an upgrade in the near future as they can grow fast)
Angelfish (their compatibility with corals and clams is often brought up and debated, outside of Geniacanthus there really is no such thing as a "reef safe" Angelfish, before purchasing one consider how difficult one would be to catch out of your display tank after it decides your corals and favorite clam are delicious, they can be model citizens but there is always a risk associated in reef aquariums)
Peter Eichler (Reef Central)
(Part I) List of Good Beginner Fish 
Fish To Be Avoided II 
jb
