Bizzare Fish Death

My remaining Scissortail Rasbora has just died in a most bizzare manner. There were no signs of illness, he/she was feeding well and showed normal behaviour. The fish was swimming around and then suddenly went limp and started spiralling to the bottom of the tank with the current. By the time I got out of my chair and approached the tank it had died.
All other inhabitants / shrimp etc are fine.
It was almost like it had been poisoned in some way. I can only imagine that something stressed it enough to kill it outright.
Gutted 

The fish was coming up to 3 years old. I believe they should should live for about 5 years.

Heart attacks have been reported in fish before. If it wasn't that, it sounds very much like something that affects fish the same way heart attacks or strokes can affect humans. The father of one of my son's friends literally dropped dead from a heart attack as he was walking with a work colleague. The post mortem said he was dead before he hit the ground. Which sounds like what happened to your fish 

Sorry to read about the sudden fatality - it must have been very disconcerting to witness but perhaps of some consolation is that it hadn't time to suffer.