Final answer:
The nitrates likely increased due to organic waste from the fish and potential fertilizer runoff, which can result in eutrophication. The process of fish waste breakdown and algal bloom decomposition consumes oxygen, leading to higher nitrate levels.
Step-by-step explanation:
The increase of nitrates after a fish has been living in the water for a couple of weeks can be attributed to several biological and environmental factors. Fish waste and decaying feed introduce nitrogen-rich compounds into the water, which can subsequently be converted into nitrates by bacteria during the nitrogen cycle.
Additionally, if fertilizer runoff is present, this can introduce excess nutrients such as nitrogen and phosphorus into the aquatic environment, fueling the growth of algae. The decomposition of these algal blooms consumes oxygen, leading to oxygen-depleted zones that are inhospitable to aquatic life like fish and shellfish.
This is also referred to as eutrophication, and the resulting lack of oxygen can reduce the ability of the area to sustain oceanic fauna.