Question

fish must have oxygen in the water to survive.
how would a decrease in oxygen affect the fish in the aquarium?
the number of fish would decrease.
the variety of fish would increase.
the variety of fish would decrease.
the number of fish would increase.

Explanation:

Fish require oxygen in water to survive. A decrease in available oxygen means many fish will not get the oxygen they need to live, leading to a reduction in their population. A drop in oxygen does not directly cause changes in fish variety, and an increase in fish number is impossible as oxygen is a limiting survival factor here.

Answer:

The number of fish would decrease.