From Nookipedia, the Animal Crossing wiki
Fishing is a large part of the Animal Crossing series. It has appeared in all the games so far, with the sole exception of Animal Crossing: Happy Home Designer . Fish appear in the museum after donating, along with bugs, fossils and paintings. They are very diverse in price, ranging from 100 bells to 15,000 bells. Not all 'fish' are actually fish, though, but rather mere aquatic species: frogs, crawfish, octopuses and lobster, for example. Fish can be found in all bodies of water, the pond, the River Pool, the river, the waterfall, the Ocean and the fish can only be caught using any of the two (in Animal Crossing and Animal Crossing: Wild World) to three (in Animal Crossing: City Folk and Animal Crossing: New Leaf) types of available fishing rods. If the player catches a fish and a villager sees them, the villager will clap.
To catch fish, the player will first need a rod. There are 3 kinds of fishing rod: the generic rod, the silver rod and the golden rod.
Where can I find fish?
Fish can be found in all bodies of water throughout the player or players' town.
This is the largest body of water in the town and is the only saltwater area. This is where most of the very expensive fish are found, including the shark family.
For a list of all oceanic fish, go to List of saltwater fish.
This is the second largest area of water in town. It runs from north to south, going through one waterfall (ACWW) or two waterfalls (AC, ACCF, NL).
Unless stated, all fish in the List of freshwater fish are river fish.
The river pool is where the river widens. It is not where the river splits in two.
River pool fish
This area is where the water cascades over a cliff. There is one waterfall in an Animal Crossing: Wild World town, but two in Animal Crossing, Animal Crossing: City Folk and Animal Crossing: New Leaf.
The pond is a small pool of water. The average town has about 3 scattered around.
What does a fish look like?
For a list of fish by shadow size, see here.
How to catch the fish
The player must not run when going after the fish or throw a snowball on water. This will scare it away, and they will not see it again. When they see the fish, they must cast the line out using the (A) button. In Animal Crossing: Wild World, it is also possible to fish by tapping the player or by tapping the water. The player should position themselves so that the float comes near the fish when it comes to a stop (change distance from shore, angle of casting, position). The fish, if it sees the bait, will attempt to nibble. Some fish nibble once then swim away, in this case you must recast the line. Otherwise, do not press (A) (or tap the screen) yet. Some Animal Crossing players keep their eyes shut and wait for the 'plop' noise, signaling a bite. When the player hears the 'plop' noise, the player must press (A), shake the remote, or tap the screen as soon as possible after the plop. In City Folk, the fish can only nibble 5 times, so if the fish has nibbled 4 times, the next nibble will be a bite. It is possible to draw the line too early, or too late. Regardless, the fish is now gone. Common fish like the Sea Bass do not need good timing, although it must be alright, in order to catch it. However, rare/expensive fish like the Blue Marlin require exact timing, so the player must keep their concentration when catching fish. Once the fish is caught the player will hold up the fish to show off and shout a catchphrase about the fish, then stuff the fish in their pockets.
The player should practice on the island, if the player is playing on Animal Crossing: New Leaf, and closes their eyes whenever the fish sees their bait. This way all the fish the player catches on the island, preferably on tours, are either all rare or the player must release them after. Also, when the player closes their eyes it helps the player concentrate more on the bait, and wait for the louder bobbing sound. After following this advice, the player should be able to catch every fish they see.
Fish that aren't fish
As stated in the first paragraph, not all fish are actual fish. These are:
Times and dates
Most fish appear all day or all year, and there are some, like the sea bass that appear all day and all year. Some fish, however, appear only at certain times of the day or at certain times of the year, like the gar. Most fish either appear all day, between 4 a.m. and 9 p.m., between 4 p.m. - 9 a.m. or 9 a.m. - 4 p.m..
Other Aquatic Creatures/Items
Diving Beetle and Pondskater
These are not fish, they are bugs. However, the Diving Beetle will dive under the surface of the water sometimes, creating a shadow similar to one of a small fish. The pondskater darts around on the surface of freshwater. It can sometimes be mistaken for a fleeing small fish. The Diving Beetle only appears in Animal Crossing: City Folk or Animal Crossing: New Leaf.
There are three items of trash. The can and boot are caught in the river, while the tire is caught in the river or the ocean. They serve no purpose. They cannot be sold to Tom Nook for a price, but he will dispose of them for free. In Animal Crossing: New Leaf, trash must be disposed of at Re-Tail for a fee of 80 bells, unless the Trash Can public works project is built.
In Animal Crossing: City Folk, villagers may lose the key to their house. When fished out, it can be returned for a reward. If it is not found fast enough, the key will stop appearing, and they will get in by themselves.