Sorry about your fish loss. I'm a fish keeper so understand what it might mean to you and your son. I'm wondering what kinds and how much vegetation, if any, there was in the pond. As you know, warm water holds less dissolved oxygen and if there are too many submerged plants (or algae), at night both fish and plants are respiring using up oxygen, which often is the cause of a pond fish kill. Even if it wasn't a quick kill, oxygen deprivation stress may lead to lower immunity followed by disease. For future reference, I would suggest having a very deep pond (less evaporative loss) with lots of emergent plants in shallow areas or floating plants and no submerged types to get around oxygen problems as well as to eliminate fish waste ammonia/nitrate build-up. Of course, as has been mentioned, shade trees or even lots of flat overhanging rocks (a dock or a boardwalk?) along the shoreline can help keep water temps down and reduce evaporation a little. Also as mentioned, a solar aerator would really help oxygen levels and water quality. I like to use medium-sized clumping bamboo (not a water plant) around the south end of a small pond as they don't drop leaves as much during the fall. A bubbling rocky stream (via a recirculating pump) into the pond is another way to get more oxygen in the water, and is helpful for birds and other critters. Finally, I'm also wondering what kind of fish you had. Food fish? Pets? Tilapia are quite tolerant of low oxygen levels, whereas any stream fish (e.g. trout) require high oxygen levels. Any fish in the carp family, including goldfish and koi, are able to gulp air at the surface. I'd follow the guidelines for fish stock density per pond surface area or volume so there's no overcrowding, which under stressful conditions will lead to disease. If you're keeping food fish, harvest as needed to keep ideal densities. Under hot or low water conditions, also reduce the population size. Lastly, just as in terrestrial permaculture we abhor monocultures the same is true for aquatic systems. A biodiverse fish and plant community is better able to withstand or recover from stressful conditions. Hope you can one day have another fish pond.