From Nookipedia, the Animal Crossing wiki
Not to be confused with the Char, a smaller fish that is sometimes referred to as the Large Char.
The Large Char is a rare fish exclusive to Animal Crossing. It is found in the waterfall pond. The fish is sold at Tom Nook's store for 10,000 Bells, one of the highest prices of all fish, surpassed only by fish that sell for 15,000 bells. It is a larger and rarer version of the Char.
It is in the salmon family, and is the second most expensive fish in that family, apart from the Stringfish.
Donating to the museum
In Animal Crossing
Blathers will say the following if the player donates the large char:
"Oh! What a large and CHAR-ming fish. That was dismal, wasn't it? Mum warned me to avoid comedy. Regardless of my shabby comedic failings, this fish will have a safe and comfortable home here."
The large char appears in the first left tank of the aquarium.
The Salvelinus alpinus or arctic char is closely related to the salmon and trout, sharing characteristics from both. It is native to arctic, sub-Arctic and alpine lakes. No other freshwater fish can be found as far north as this fish. It is a rare fish in Britain, only found in glacial lakes but can be the only fish to be found in lakes around the Canada Arctic. In Scandinavia, it is extremely common and fished extensively. The color of the fish varies between pale pink and bright red. It is an alternative fishing solution to Rainbow Trout for a farming fish because it uses very little resources. Char feed mainly on insects, but can feed on fish and small crustaceans. It is a larger version of the Char.