Christmas tree at aquarium in Fukui Pref. Dec. 04 FUKUI, Japan Breeding staff wearing Santa Claus costumes set up a 3-meter-high Christmas tree in a water tank at Echizen Matsushima Aquarium in the town of Mikuni, Fukui Prefecture, on Dec. 4, 2009. Some 3,000 fish of 20 kinds swim in the tank, which measures 10 meters in width and 6 meters in depth. (Kyodo) The Oxford Advanced Learner’s Dictionary says: fish noun (pl. fish or fishes) [HELP] Fish is the usual plural form. The older form, fishes, can be used to refer to different kinds of fish. Therefore, shouldn't fish be "fishes"?