Investigators from the agriculture ministry in the southern state of Santa Catarina found local businesses selling adulterated pollock, hake and other imported species, police said in a statement.
The fish had been injected with "water and chemical products, which among other effects raised the price of the products," police said. The fish were then routed by corrupt agricultural inspectors to avoid the usual controls.
The adulteration took place while still in China, said federal police representative Mauricio Todeschini.
In March, Brazil's huge meat export industry was hit by revelations that 21 Brazilian processing companies had been using chemicals to make old meat look fresh.