These were my fish last year; the three big ones in the centre of the picture are amongst those I've lost in the last few days. The golden orange one was almost 25 years old and one of my very first pond fish.
Coldwater fish do not like this weather. It is well known that water gets starved of oxygen in warm, sticky, stormy weather. This last few days has been especially bad; I don't remember anything so hot and humid for many years. and of course it hits the largest fish hardest. They have a larger body mass to support, and their body mass to gill are ratio must be higher than in smaller fish. So the big fish get hit first. But then I do also partly blame myself as my pond maintenance has probably not been up to scratch recently.
It's a timely reminder not just of the fragility of life but of the old adage about fishkeeping: We are not fishkeepers, we are water keepers.
Fortunately it has been noticeably cooler today, and the forecast is for it to get cooler through the weekend and for rain from Sunday; both of which will help.