Sudden death

I had a large approx. 7 inch orange goldfish living in my tropical fish tank. It was about 8 years old. Yesterday morning I fed everyone and all were alive and happy. Two hours later the goldfish was dead. He wasn't sick. The day before I fed them some boiled lettuce which I do occasionally. Could it have been a heart attack or blockage?

other fish were much smaller swords,siamese algae eaters,platys and a medium size goldfish. Water was slightly alkaline. Dont have a kit to test other parameters. Tank is 120cm long. There were no visible abnormalities with the goldfish and no one else died.