But they are FILTHY! Goldfish actually seem to excrete more than they eat . . . within a day or so there will be nasty tubular-shaped goldfish deposits all over the place. They clog filters, especially the undergravel ones, and you have to wash their gravel frequently. They'll also tear up any plants you put in the aquarium.
Goldfish belong in an outdoor pond where the fish by-products are spread out over a larger area and you don't have to stare at it all day. Dig it deep enough and put water lilies and other aquatic plants in pots on the bottom, and you'll never be bothered by goldfish poop. (Dig a shallow inclined beach on one end of your pond and decorate it with washed and dried seaweed in the spring, and you may get lots of little baby goldfish too!)