Nothing unusual about a gold fishing dying. We’ve had goldfish die before but never suddenly, without symptoms or change of environment or something.
Situation; two fancy goldfish in a aquarium. Three years old. Been in the same environment for two years. Gave them food at 2pm and they both ate voraciously as usual. At 4pm one was dead. The other has been just fine now for a couple of weeks.
Just curious. What would you conjecture as possible cause of death.
Causes of sudden goldfish death: cats? 
Seriously, is three years old for a goldfish? The one may have died of old age. Was there a change in the environment (water temperature, salinity, etc)?
Maybe it just had a heart attack.
Could it be… Murrrrrderrrrrr?!? Is the remaining fish able to look you in the eye and say he didn’t do it?
Seriously, goldfish can live a long time, but on the flip side you’re often never really certain how long they’ve been living at the pet store. And I imagine there are many potential causes of sudden death. I wouldn’t think they’re immune to heart attacks or strokes or what not.
The longest I’ve had one live was 13 years. I’ve read they can live as long as 20 or more.
Our first work fish (female betta fish) had a stroke overnight - at closing she was perky and happy, next morning she was all paralyzed down one side.
Something I had never really thought of before that day, but a half-paralyzed fish is really sad.
She held on for a good three months afterwards, but I could easily imagine a stroke doing worse, with no signs beforehand.
Clearly suicide.
(I used to be an advisor on a koi & goldfish care board so pardon me while I launch into Fishy Interrogator Mode 
)
I’m not quite sure what you mean by the other fish “being fine for weeks” but they’ve been in the same environment for 2 years now. Was the other fish sick or something?
Have you done any big water changes lately? Different dechlorinators/ other water treatments? Checked the pH, ammonia etc. to see if anything’s gone wacky with your water chemistry? Stuff like that can send a fish into shock, which can kill them before you even realize they were sick.
Unfortunately fancy goldies just aren’t the most durable of critters, they can get hit with everything from neurological disorders to organ failure to bacterial infections. I swear sometimes they just drop dead on a whim, just so they can swim around fish heaven and laugh at us :mad:
I interpreted it as being that on the day that the fish died (two weeks ago) it ate at 2pm and died at 4pm, rather than that it died within the past day or so.
I’m no expert, but I’ve heard that “dead” goldfish frequently aren’t. Goldfish will float on their side at the top of the tank not moving if they are constipated. This frequently happens shortly after a meal, if the owner feeds them incorrectly.
I rather like the "I swear sometimes they just drop dead on a whim, … ". The two had been in the same environment for two years. One died and one has not.
We did isolate it for about 12 hours to examine it to see if it were indeed dead. Armature necropsy showed nothing.
Just curious as to what your comments might be. Thanks.
I put a little ‘rescued’ goldfish into our big tank a year ago, and today it’s entree sized (would make a great Lean Cuisine frozen dinner). We’ve had other big goldfish in the past who seemed fine for a few years and then just…exploded. yes, they just exploded, within a few days of each other. I am not well versed in the ailments of goldfish, but that’s what happened. Goldfish I found are pretty tough, but there are plenty more where they came from, cheap, in any pet store.
I thought I was putting a long-term, slow-release food tablet int the water when we left for the weekend. Instead it was a tank-cleaning tablet. Fish all dead.
SGDS. Very sad.