The same thing happened to me, I have only had my goldfish for two days, they were a present from my brother, I noticed that 2 of them didnt really mingle with the others as much and were lying on the floor last night every so often they would turn on to their side, and sadly they were both dead when I got up this morning.
I agree that they may only be fish but they do have their own little personalities.

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Mummy to 2 babies, one has 2 legs and the other has 4 xx