My Goldfish Died!

But why? The fish was a fairly standard goldfish with a nice comet type
tail. It was about two years old and shared an 18x12x12 tank with 5
Whitecloud. I've been feeding with Tetrafin.

The goldfish "jerome" has never exhibited any disease symptoms, no
whitespot, no finrot, no slime disease, no clamp fin. All the Whitecloud
have always demonstrated good health.

Over the last few weeks Jerome started to slow down and stopped feeding over
the past couple of days and is body (not fins though) started to droop.
Yesterday morning the fish was curled up on the bottom of the tank barely
alive. The gills were nice and bright. Now Jerome is dead.

I've not come across this before - and take his as a knock to personal pride
in my husbandry!

The Whitecloud are all perfectly fine exhibiting none of the symptoms.

Any ideas what this was?

TIA

Steve