Final answer:
The presence of a fish in water with bromothymol blue can lead to the production of CO2 from respiration, which may turn the water yellow if enough CO2 dissolves to form carbonic acid, lowering the pH.
Step-by-step explanation:
If we put a fish in the water with bromothymol blue, the color of the water could potentially change because bromothymol blue is a pH indicator. Normally, it is blue in neutral water, but it turns yellow in acidic conditions. When a fish is added to the water, it will respire, producing carbon dioxide (CO2), which can dissolve in water to form carbonic acid, thereby lowering the pH of the water.
If the pH drops sufficiently due to the accumulation of CO2, the bromothymol blue solution would likely turn yellow. However, a single fish in a reasonably-sized volume of water may not produce enough CO2 to alter the pH significantly, so the color change might not be immediate or noticeable without a significant number of fish or over time.