鯉 (carp) and 鮒 (crucian carp) are both common freshwater pond fish that look similar to non-experts, but 鮒 is smaller and lacks the barbels of 鯉.
Japanese crucian carp (cultured variety)
crucian carp caught in midwinter
Carassius buergeri; a species of crucian carp
silver crucian carp; Carassius langsdorfii
cowardly, good-for-nothing samurai from the countryside
Usually written in katakana; usually written in kana.
Kanji form; rarely used in modern writing.
Hiragana spelling, also common.
Alternate kanji form; very rare.
The origin is uncertain; the word is native Japanese. The kanji 鮒 and 鯽 are Chinese characters for the same fish.