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From The Aquarium Wiki

Gymnotidae, or Naked-Back Knifefish, belong to the order Gymnotiformes. They are a group of Knifefish that are found only in fresh waters of Central America and South America. All have organs adapted to the exploitation of bioelectricity. The family has 33 valid species in two genera. There are a number of undescribed species known in museum collections.

	Common name	Difficulty	Maximum Size	Minimum Tank Size
Electrophorus electricus	Electric Eel	Difficult	98.42519685039498.425 in250 cm	200200 gal757.082 liters 757,082.356 mL
Gymnotus tigre	Tiger Knifefish Macana Tigre	Moderate	1818 in45.72 cm	9090 gal340.687 liters 340,687.06 mL