From Nookipedia, the Animal Crossing wiki
The Angelfish (エンゼルフィッシュ, Enzeru fisshu), Pterophyllum altum, is a rare fish with coloration similar to that of the Barred Knifejaw. It can be found in the river and appears between 4 P.M. to 9 A.M. from May to October. It sells for 3,000 Bells and has a tiny shadow size.
Donating to the museum
In Animal Crossing
Blathers will say this upon donation to the museum:
"Gorgeous. Sim-ply gorgeuous. Heavenly, even. We will treasure your gift. I give you my word as an honest, upstanding owl."
It is found in the bottom-right tank, grouped with other rare fish.
In Wild World
It can be found in the second tank after donating it to museum. Upon donation, Blathers the curator will say this:
"The angelfish is called the queen of tropical fish, and quite rightly so, eh wot! Indeed, this one should bring a ray of sunshine to this dusty old museum! However serene these fish may look, they're actually rather feisty...Every rose has its thorn, isn't that what they say?"
It is found in the second tank in the aquarium after donation.
In City Folk
Blathers the curator will say this:
"The distinctive shape of the angelfish lets it appear larger than it actually is. ...It's quite beautiful, and though it is a tropical fish, it has many tricks for staying alive, eh wot? Those skills make it far more resilient than other, pampered tropical fish. Perhaps this fact is what earned the angelfish the title "Tropical Fish Queen."
It can be found in the bottom-left tank with other small river fish.
In New Leaf
An information board in the aquarium will display this description of an Angelfish after donation:
"These tropical fish move very gracefully because of long fins on their backs and underbellies. Males and females are almost impossible to tell apart, making identification tricky. They lay eggs among water plants. Both genders look after the babies while they cling to weeds. The young look like any other fish, but they gain their angelfish looks once they leave the weeds."
The Pterophyllum altum, or Altum Angelfish, as they are also known, grow larger than how they are represented in the game, in fact reaching over 50 cm in the wild (the size of a small barred knifejaw). It originates from South America, in the river basins of the Amazon River, Orinoco River, and Essequibo Rivers. They are ambush predators, meaning they hide behind roots and attack on small fish when they least suspect it. Their small bodies and striped coloration makes it easy for them to hide. They are monogamous, which means they only have one partner throughout their lifetime with which they lay eggs. It likes weak acidic water, with a pH of about 4.5 to 6.
More information on this topic is available at Wikipedia.
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