It may shock some people to learn that goldfish can live twice as long as a dog. They’re just one of several fish species ...

But in your home tank, there's no bacteria to be found. You have to introduce it, and that's where a lot of goldfish owners slip up because you can't just buy a fish the same day you get your tank.

It won’t shock anybody that a koi fish needs a much bigger space than a goldfish or guppy. A 20-gallon tank is suitable for a single goldfish. Yet, these fish can quickly outgrow their aquariums ...