We often catch juvenile carp (Cyprinus carpio) in lower central valley rivers, but they are typically a golden brown color. However, this one appears to be aspiring to be a Koi. Carp are native to Asia, but have been introduced to almost all parts of the world. They have been bred for bright colors and patterns as koi in Japan since the 1820s. Now many varieties are available worldwide. Unfortunately, for this little fish, bright coloration will likely make it a target for predators in a river environment.
Photo source: FISHBIO