There are approximately 27,000 known species of fish, easily making them the most diverse group of vertebrates, and scientists estimate that there are still thousands of fish species yet to be discovered!

The ocean sunfish (Mola mola) produces more eggs than any other species of fish. In fact, it produces more eggs than any other vertebrate on the planet. A single female can produce up to 300 million eggs at a single spawning, each measuring about 1.3 mm (0.05 in) in diameter! Considered a delicacy in Asia, a single mola can fetch prices as high as $600 (U.S.).

There are over a hundred different varieties of goldfish, classified by color, body shape, finnage and outgrowth of the eyes. Some common varieties of goldfish include the Comet, the Fantail, the Oranda, the Shubunkin, and the Black Moor. Perhaps the most popular of all aquarium fish, goldfish have been known to live up to 25 years!

Although it is, of course, extremely difficult to measure the swimming speed of large fish in the wild, the cosmopolitan sailfish (Istiophorus platypterus) is considered by many experts to be the fastest fish in the world. It has been clocked at speeds of 110 km/h (68 mph). The average fish would be hard-pressed to reach 20 km/h (12 mph).

The walking catfish (Clarias batrachus), also known as the magur or pla duk dam, caused a panic among fish farmers in Florida when it began invading their aquaculture farms and feeding on fish stocks. Dubbed "Frankenfish" by the media, the walking catfish breathes air and is capable of making short migrations across land. When their food supply runs out, the walking catfish simply migrates, marching like an army, sometimes thousands at a time, to the next food supply. When they stumble upon a fish farm, the effects can be devastating as they are very aggressive predators. If they go undetected, they can wipe out a farmer's stock of fish in a relatively short time. Even killing a pond by pumping down, spreading active lime, and using chlorine is not guaranteed to stop the spread of these fish as they are quite capable of burying themselves deep in the mud and resurfacing when conditions are more favorable. A native of Southeastern Asia, the walking catfish is now established in Florida and has been reported in numerous other U.S. states including California, Connecticut, Georgia, Massachusetts, and Nevada. In response to the walking catfish, fish farmers have begun to erect barrier fences to protect their ponds.

The most poisonous fish (to eat) is the puffer fish whose ovaries, eggs, blood, liver, intestines and skin contain a fatally poisonous toxin called tetrodotoxin. Less than 0.1 g (0.004 oz) of this toxin is enough to kill an adult in as little as 20 minutes. Ironically, the poisonous puffer fish is considered a delicacy in Japan. Although the poison is generally removed before they are served, eating puffer fish can still be fatal if the fish was improperly prepared. In some countries, a chef must be certified in order to serve puffer fish because the poisonous parts of the puffer differ slightly from species to species (there are about 120 different species of puffer fish). About 100 diners die each year after eating puffer fish.

In the movie Finding Nemo, Nemo is a clownfish (Amphiprion ocellaris). Because of the dangers of reef life for such a small, brightly colored fish, clownfish in the wild tend to develop a relationship with sea anemones (clownfish are one of the few species that can avoid the stings of these anemones) which provides them with some added protection. In a safe aquarium environment, however, this relationship is unnecessary. Due to the popularity of Finding Nemo, public demand for pet clownfish nearly tripled after its release.

Reaching an average length of 45 feet and weighing up to 15 tons, the Whale Shark (Rhincodon typus) is generally considered the largest species of fish in the world. One specimen captured in the Gulf of Thailand in 1919 measured an incredible 59 feet! Not related to whales (which are mammals, not fish), the Whale Shark probably earned its name because of its abnormally large size as well as the fact that, like whales, it is a filter feeder, swimming with its mouth open in order to suck up plankton and small sea creatures.

Measuring only 7mm (about the width of a pencil), the stout infantfish holds the record for world's smallest fish. It lives around Australia's Great Barrier Reef and is thought to have a lifespan of only two months. Prior to the discovery of the stout infantfish, the dwarf goby fish was generally considered the world's smallest fish.

When a shark loses a tooth, a new tooth grows in to replace it. In fact, below each tooth (inside the gums) are up to 7 layers of replacement teeth, just waiting to grow in. Some species of sharks have been known to shed as many as 50,000 teeth during their lifetime!