So how did we really get the most common ornamental fish in the world?  Scientists are working to figure it out. This article is for all you research lovers!
 
Goldfish (Carassius auratus var.,GF; 2n = 100) and red crucian carp (C. auratus red var., RCC; 2n = 100), are the most prevalent ornamental fish in the world, and these species belong to Cyprinidae (family), Cyprininae (subfamily), and Carassius (genus) (Luo et al., 1999; Wang et al., 2014). GF and RCC are considered varieties of crucian carp (Carassius carassius). An obvious difference between GF and RCC is that GF has distinct split double tails (twin tail), whereas RCC does not. Although some studies have suggested that GF evolved from crucian carp (Podlesnykh et al., 2015), the direct evidence of its evolutionary pathway is lacking. 
 
Frontiers | The Formation of the Goldfish-Like Fish Derived From Hybridization of Female Koi Carp × Male Blunt Snout Bream | Genetics (frontiersin.org)