As mentioned, water quality is always suspect.
Your fish looks pretty healthy, I assume it is a dead one? It's tail fin looks a bit ragged, but not bad at all. Nothing to suggest cause of death.
I'm new to all this as well...82 degree water temps sounds high especially this early in the year, but what do I know about Texas norms? Should you be looking into just aerating at night? I know that the goal is to turn the pond over at least once a day, but can it be turned over too much? The Pondlyfe 2 system should be turning you small pond over many times a day. At what depth is your diffuser"?
Could these loses have something to do with spawning. I know in the aquarium world attitudes of breeding/parenting fish can change quickly and have some casualties. Not to mention the stress of not eating during the spawn.
Just my thoughts...I wouldn't worry too much about a few fish showing up dead or weak, but it is a good steward to be concerned.
