Fish To Be Avoided: (fish with incredibly low survivability in aquaria)
Moorish Idol (a few success stories but miniscule amounts live long, difficult feeder, mystery deaths, and even when feeding often slowly starves)
Holacanthus tricolor [Rock Beauty] (nearly impossible to meet the dietary needs in home aquaria)
Centropyge heraldi (almost always caught using drugs)
Centropyge multifasciatus
Centropyge venusta
Clown tang (VERY ich prone and a finicky eater, horrible survival rates, when they do live they can be quite mean)
Clown Sweetlips
Oriental Sweetlips
Platax pinnatus [Pinnatus Batfish] (gorgeous fish when young, very very few success stories)
Platax batavianus [Tiger Tiera Batfish]
Orange Spotted Filefish (specialized coral polyp feeder)
Most Butterlyfish (except those listed below)
Ribbon Eels (rarely eat in captivity)
Snake Eels
Garden Eels
Cleaner Wrasses (specialized parasite feeder, leave them in the ocean where they can do their job)
Anampses sp. Wrasses (VERY poor shippers and need tanks with their special needs in mind, even then they often perish)
Leopard (Macropharyngodon) Wrasses (see above but there are more success stories, must be kept in reef aquariums)
Pseodojuloides Wrasses (very dificult to keep alive, they almost always die in transit so you don't see them very often if ever)
Parrotfish
Tilefish (VERY timid and difficult to get to eat, also excellent at carpet surfing)
Rays
Grunts
Jacks
Drums
Trumpetfish
Remoras
Chambered Nautilus ( a plethora of reasons to leave them in the ocean, not a single good reason to add one to an aquarium)
Fish Best Left For Experienced Hobbyists:
(finicky nature, parasite prone, specialty feeders, require specialty tanks or threatened species)
Anthias (some can do fairly well when kept with less aggressive fish, others require special tanks with their needs in mind)
Twinspot Anthias (the anthias here are borderline, but I feel the overly finicky and difficult nature of these two deserve special notice)
Square Anthias (collection methods and stress after collection seem to be especially problematic)
Platax tiera (can ve hardy once acclimated but can be problems feeds, stress easily, and are disease prone)
Regal Angelfish (Red Sea Specimens tend to be hardier and more willing to accept prepared foods and the more recent trend to keep this fish in reef aquariums helps with survivability)
Bicolor Angelfish (concerns with drugs used in collection and unwillingness to accept prepared foods)
Genicanthus sp. angelfish (hardy once acclimated, but lots of problem specimens due to the depths they are usually caught at)
Garibaldi (coldwater species and protected)
Trunkfish [Box and Cowfish] (most are rather sensitive and can release toxins when stresed or dying)
Clown and Gumdrop gobies (poor shipper, once established a good surviver with less boisterous fish)
Catalina gobies (coldwater species that will not do well longterm in tropical temps)
Mandarin dragonettte (requires large amounts of live food typically which can be provided naturally in 50+ gal. tanks with a good amount of live rock)
Radiata Lionfish
Fu manchu Lionfish (All the dwarf Lions require something bordering a species tank, the later two are also very sensitive)
Dwarf Zebra Lionfish
Anglerfish (many get very large and can consume fish enarly their own size)
Acanthurus sp. tangs (ich prone, Achilles, Powder Blue, Powder Brown, and Goldrim are rather finicky and beginners should be especially leary)
Ctenochaetus tangs (ich prone, some of the hardier tangs once established, the Chevrom being the least hardy)
All Butterflyfish (except vagabond, longnose, Heniochus, golden, pebbled, Klein's, Lemon, Auriga and Racoon, which need large tanks)
Seahorses (need species tanks and special diets)
Seadragons (very rare and I'm unaware of and longtern success)
Pipefish (see Seahorses)
Anthias (require a good amount of swimming room, peaceful tankmates, and frequent feedings, often unhealthy and starving by the time they make it to dealers tanks)
Longnose hawkfish (jumper and be careful with ornamental shrimp)
Octopus (must have species tanks, lots of swimming room, and should probably be on the above list)
Cuttlefish (similar to Octopus, but slightly higher success rates)
Squid
Porcupine pufferfish (very disease prone)
Fairy wrasses [Paracheilinus and Cirrilabrus sp.] (require peaceful tanks and do best in reef aquariums, they stress and the first few weeks in captivity will often make or break their longevity)
Leopard Blenny [Exallias brevis] (specialized coral feeders)
Scooter Blenny (see Mandarin Dragonettes)
Lawnmower Blenny (will sometimes not accept prepared foods and will starve to death in tanks without a natural algae food source)
Sandhopper
Sleeper gobies (Valenciennea sp.) (Often starve to death even when accepting prepared foods, tanks with large sandbeds containing lots of food will help as will frequent feedings when they will eat, mated pairs may help)
Rainford's Goby (often will not accept prepared foods, need established tanks with a sandbed full of life)
Twinspot Goby (mated pairs in large tanks with established sandbeds will help)
Bangaii/Borneo Cardinals [Pterapogon kauderni]
Fish That Require Huge Aquariums (200 gallons or more)
Sharks (require much larger than 200 gal. and should just be left out of home aquaria, Nurse sharks can grow to 14ft. long!)
Most Groupers (especially take note of the cute little Panther Groupers)
Snappers (those cute little Red Emperor Snappers get big)
Naso sp. Tangs (Many will even outgrow common sizes like 125 gal. aquariums)
Moray Eels (large species)
Soldierfish
Orbi and Spade Batfish
Twinspot wrasse (Coris aygula) (beginners take special note of these three as they're often offered as cute juveniles, they get very large and very mean)
Red coris wrasse
Dragon wrasse
Flounder
Tassled filefish (often offerer when cute and tiny)
Many Large Angels (when purchasing any angelfish that isn't Centropyge be sure to check their ultimate size: take special note of the FRench, Gray, Blue, and Queen whcih are often offered as cute little juvenilles)
Venomous Species
Stonefish (can be deadly)
Lionfish
Rabbitfish
Scorpionfish
Coral Catfish (these also get up to a foot long and no longer school once larger)
Bluering Octopus (can be deadly)
Toadfish
Extremely Aggressive Species
Undulated Trigger (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 Trigger (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 takeover tanks)
Damselfish (they're not all bad, but once for once 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., frequent feedings of meatier foots can lead to success)
Christmas Tree Worms (filters feeders that rarely live long in home aquaria)
Goniopora sp. (some strides have been made but still miserably low survival rates)
Feather stars (require huge amounts of flow and large amounts of tiny planktonic organisms)
Basket stars
Crown of thorns (duh
)
Linkia stars (disease issues and poor acclimation to aquarium life, problem feeders as well)
Wild small-polyped scleractinian (SPS) corals 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)
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 and very poor survival for 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 few exceptions)
Flame Scallop (filter feeders that usually waste away in home aquaria)
Anemones (most anemones should be placed in specialty tanks and also have very poor survival rates)
Bright yellow anemones (dyed; and done most commonly with Sebae, but also seen with Long Tentacle and Carpet anemones far less frequently )
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
Organ Pipe 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 sugger 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)
Tangs (should have larger aquarium to provide them plenty of swimming room, no you a tang is not suitable for your nano cube or 29 gallon tank, when small 3'-4' aquariums can be suitable though not recommended by many, just be sure you're planning an upgrade in the near future as they can grow fast)
Zoanthids (some of these can contain Palytoxin which can be quite dangerous and make you very sick, it's not always the case 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)