From Nookipedia, the Animal Crossing wiki
Donating to the museum
In Wild World
"When I hear loach, I think of a friend of mine who makes smoothies of them. It sounds repellent, I know and to be honest, it is. Hop I say! The stench!"
In City Folk
"The loach is a slippery, somewhat vile-looking fish...but in truth, it can be quite useful, wot? For ages, some cultures used the loach as a poison treatment. It also tastes great in a hot pot... A bit of wisdom for the day, eh wot? One must never judge a person, or a fish, by its appearance..."
In New Leaf
"Loaches are rather small fish that make their habitats in rivers and muddy water. They have 10 whiskers around their mouths with taste buds built in for tasting sweetness and bitterness. Loaches can breathe through their intestines as well as their gills―a rather unique quality of the species. They push their mouths above the surface of the water to inhale, exhaling through their tail regions. Making stew with loaches and burdock root is known to improve stamina, or so people believe."
The Loach can be found in the leftmost tank of the freshwater fish room.
It is unclear what type of loach appears in Animal Crossing, since there are vast varieties of loaches. It is, however, a member of the Cobitidae family of loaches, called the True Loaches, which live in fast flowing waters. All true loaches are "bottom feeders," meaning they swim at the bottom of rivers and tanks eating the algae at the bottom. They also eat small crustaceans and insects, but only during the day. Under true loaches, it can be a horse-face loach which grows to roughly 20 cm (8 inch). This type of loach is a private and public aquarium favorite because of its eating habit which cleans a tank for the owner, making it easier to keep fish.