Hot summer alone isn't enough to kill the fish unless the tank was placed near a window where it can receive direct sunlight. On the other hand, the fish might die to sudden change of temperature if the water temperature was changed too sudden. Since fish are cold blood animals, they can't regulate their body temperature. Sudden change of temperature can shock them. One of the most common causes of fish deaths is when the fish owner change the water in the tank with too cold or too hot temperature. From my own observation on tropical fish, if the new water has a 6F or great difference compare to the tank water, the fish can get into shock. Goldfish might have a higher tolerance than tropical fish, but it can't be too different.
If the summer is hot there, then the tank water might be quite warm too. The tap water might not be as warm since the pipes often run underground where it is much cooler in the summer.
Have you done anything recently before the fish was dead?
Such as a change of water, or cleaning of the tank, or cleaning of the filter?
