For seven years, Bani Akter, a villager in Horingachi, Kushtia District of southwestern Bangladesh, had worked in moneylending to supplement her farming income. “That business,” she said, “caused us a lot of trouble, illnesses and constant worries.”

But everything changed two years ago, when Akter took a leap of faith and turned to raising fish.

Akter has two sons and a daughter, all of them in school. On 1 bigha of land (about 0...