I have a 20 gallon tank filled with Java ferns and small rocks and crystal clear, filterized water, and a 13" goldfish, 3 zebra danios, and a Lyretail molly. Before the problems happened, I had 1 goldfish, 4 zebra danios, 2 Lyretail mollies, a male guppy, and a Silverfin Shark. I lost the Shark because it was apparently sick when I bought, as was the molly, but the guppy and a danio had a more mysterious case. I feed my fish every day twice a day in the morning anf the afternoon. My danio and my guppy disappeared out of nowhere with no signs of illness. I didn't see any bodies of dead fish under or on the rocks or ferns. I checked my filter and there were no strange signs of dead fish. I disregarded it, but now im wondering, because the goldfish usually lingers at the bottom looking for food, could my goldfish have gotten hungry and resorted to cannibalism on my guppy and danio? It's mouth is big enough to barely fit a danio or guppy it's size in, but ive never seen my goldfish munch or even try to catch another fish in my tank. Could my goldfish have eaten the remains of mthe fish if they died from sickness?

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By the way, my father has a 75 gallon tank that we raised the goldfish in, but we decided to move it because it was being a food hog and it dug up all of the fern lol

Thanks

The goldfish will kill the other fish and also itself because the tank is way too small for a goldfish. It's waste products will toxify the water and overload the filters. A large goldfish will eat small fish while they are sleeping or hiding. And of course when they are sick or dead.

It is not cannibalism for one species of fish to eat another, any more than it is cannibalism for one species of mammal to eat another. Humans (some of them) eat beef and pork, and genetically those are far closer relatives than different species of fish can be.