Разг.-ласк., уменьш. от «рыба»; также как ласковое обращение.
little fishDiminutive, affectionate (about a fish). В ведёрке плескалась рыбонька.
A little fish was splashing in the bucket.
sweetheartAffectionate form of address (often to a woman or child). Рыбонька, не переживай, всё будет хорошо.
Sweetheart, don't worry, everything will be fine.
small fishNeutral; less affectionate than the Russian diminutive. darlingAffectionate form of address. dearAffectionate form of address; can sound slightly formal depending on context.