(Repost with Permission) - Peter Eichler (Reef Central)
Fish To Be Avoided:
(fish that have incredibly low survivability in aquaria or are totally unsuitable for home aquaria)
Moorish Idol (a few success stories but miniscule amounts live long, difficult feeder, mystery deaths, and even when accepting prepared foods 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 (this and the venusta are very similar, they don't adapt to aquarium life well, and both seem particularly prone to Lymphocystis)
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(difficult feeders and get quite large)
Oriental Sweetlips
Platax pinnatus [Pinnatus Batfish] (gorgeous fish when young, very very few success stories, diet and disease are big issues)
Platax batavianus [Tiger Tiera Batfish] (see above)
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 feeders, 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 sensitive, 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)
Sharks
Rays
Skates
Grunts
Jacks
Drums
Trumpetfish
Remoras (unless you have a large Shark or Whale in your backyard oceanarium probably not a good idea)
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 Or Knowledgable Hobbyists: (finicky nature, parasite prone, specialty feeders, require specialty tanks, or threatened species)
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, some almost require special tanks with their needs in mind)
Twinspot Anthias (one of the more difficult standouts of the Anthias)
Square Anthias (collection methods, stress, and starvation after collection seem to be especially problematic here)
Platax tiera (can very hardy once acclimated but there can be problems feeding, they 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 frequent unwillingness to accept prepared foods)
Genicanthus sp. angelfish (hardy once acclimated, but lots of problem specimens due to the depths they are collected at, take extra special care in examining and observing them before purchase)
Garibaldi (cold water species and protected)
Trunkfish [Boxfish 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, will only rarely accept prepared foods)
Radiata Lionfish
Fu manchu Lionfish (All the dwarf Lions require tanks with their needs in mind, the later two are also very sensitive, very shy, and poor shippers)
Dwarf Zebra Lionfish
Anglerfish (most get very large and can consume fish nearly their own size)
Acanthurus sp. tangs (ich prone and fairly sensitive to water conditions, Achilles, Powder Blue, Powder Brown, and Goldrim can be rather difficult and beginners should be especially leary)
Ctenochaetus tangs (ich prone, some of the hardier tangs once established, the Chevron is probably the least hardy of the genus)
All Butterflyfish (except Vagabond, Longnose, Heniochus, Golden, Pebbled, Klein's, Lemon, Auriga, and Racoon, which need large tanks)
Seahorses (need quiet species tanks and large quantities of nutritious live food)
Seadragons (very rare and I'm unaware of any longterm success)
Pipefish (see Seahorses)
Longnose Hawkfish (hardy fish but they are notorious jumpers and be careful with ornamental shrimp with all Hawkfish)
Porcupine Pufferfish (can be hardy but also seem very disease prone)
Fairy wrasses [Paracheilinus and Cirrilabrus sp.] (require peaceful tanks and do best in reef aquariums, they stress easily 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 Gobys (Valenciennea sp.) (sometimes 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 as well)
Rainford's Goby (often will not accept prepared foods, need established tanks with peaceful fish a sandbed full of life)
Twinspot Goby (combine the suggestions above for this one)
Cephalopods (not fish, but including them here because of their intelligence compared to the dumb lumps of goo that are most invertebrates, the Nautilus from above is in this group as well)
Octopi (must have species tanks, lots of swimming room, and should probably be on the above list)
Cuttlefish (similar care to Octopi, but slightly higher success rates)
Squid
Fish That Require Huge Aquariums (200 gallons or more):
Sharks/Rays/Skates (require much larger than 200 gal. and should just be left out of home aquaria, Nurse sharks can grow to 14ft. long!, repeating this one so it sinks in)
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 but grow large)
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)
(cont: Fish to be avoided Part II) List of Good Beginner Fish 
Fish To Be Avoided Part I 
