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Most fish use oxygen dissolved in water for respiration and metabolism.

However, solubility of gases decreases with increase in temperature. Hence, as the water gets warmer, less oxygen begins to dissolve in water, making it hard for the fish to breathe.

Also, as temperature increases, the fish's metabolism increases, in turn increasing the oxygen requirement. Eventually, there won't be enough oxygen for the fish, causing it to suffocate and maybe die.

Hence, if you have an aquarium, you should always check and maintain its water temperature. You can also use a filter or bubbler which helps add and circulate oxygen, thus maintaining your fish's health.