Final answer:
The main factor that affects the amount of dissolved oxygen in tank water is temperature. As temperature increases, the solubility of oxygen decreases.
Step-by-step explanation:
The main factor that affects the amount of dissolved oxygen in tank water is a) temperature. Temperature has a direct impact on the solubility of gases, including oxygen, in water. As temperature increases, the solubility of oxygen decreases, leading to lower levels of dissolved oxygen in the tank water. This can be detrimental to the health of aquatic organisms, as they rely on dissolved oxygen for respiration.