Yeah a goldfish...
I have 5 (now) Comet feeders and one fancy (2 tails) with 2 juvenile Koi in a small outdoor pond, came home yesterday and one of the Comets was on the waters surface and very lethargic...It came to me instead of hiding like normal...
I caught it easilly and put it into a 5 gallon bucket with water from the pond (like a 75 or 100 g preformed) and put in a sponge filter that I am cultivating in my main tank...He lasted another 4 hours before he stopped moving. I made the effort to help, but it didn't make it.
Still not sure what happened, other than being dead there were no external issues (ie: rot or missing scales)and my water parameters are all good (0 ammonia, 0 Nitrites, ~20PPM Nitrates, PH 7). All the others are fine and eating well.
As a just in case, I pulled some dead lilly leaves and quite a bit of the watter lettuce thats growing on the surface, out and performed about a 30% water change..Water is still clear, but the filter will also get cleaned tonight. Like I said, I dunno...