All fish nitrogen into the water as ammonia and organic nitrogen. These chemicals are byproducts of their metabolism and respiration.

The correct answer would be - release/excretes.

Explanation:

In ponds, rivers, or similar ecosystem nitrogen cycle is completed with the help of the fishes. In this ecosystem, the nitrogen cycle initiates with the eating plants or fish food by the fishes and performs metabolism and cellular respiration which produces byproduct ammonia and organic nitrogen as animal waste.

This waste is released or excreted by the fish in the water where the beneficial bacterium helps in restoring nitrogen back to the atmosphere by breaking these wastes.

b brown mice / white sand dunes