Goldfish death

Question
Hello. Recently, one of my two goldfish died a few days ago. The reason is unknown as they were healthy and had been living together for almost 3 years. The disappearance of the second fish seems to have affected my other fish. The other one is less active now, doesn't move very much around the tank, and often floats aimlessly in the water. The fish gets very excited when I come to feed it, but other times I have observed it staying still and exerting minimal energy. Do you have any ideas to solve this problem? We have introduced other goldfish into the tank before, but none lasted more than a few days, as they were killed by the other two fish. Should I try to add new species of fish or get a new tank with different surroundings? Is the fish somehow affected by losing its companion after so many years? Thanks for helping!

Answer
Hello,

Goldfish are very social creatures by nature, and benefit greatly from company. It is quite possible that your fish is lonely. Goldfish need to be kept with ONLY other goldfish, and not mixed with any other species. You could try adding more goldfish to the tank and see if your surviving fish improves.