Answer:
C. Increasing the amount of oxygen in the tank
Step-by-step explanation:
If the fish are struggling or we see that they are going near the surface to breathe, that could indicate that there is not enough oxygen for the fish to breathe easily at all depths of the aquarium tank. By increasing the amount of oxygen present inside the tank we will assure that the fish have enough oxygen to live well. Not too much since too much oxygen can be bad for the fish.