Inside this clear egg, something moves like liquid gold.
(no dimorphism)
Spoiler
Mirrorfin hatchlings require a lot of time and practice to master illusions, starting only with amorphous blobs. While a small amount of gold is with them in the egg, they will burn through it quickly and need to find new sources on their own. Those seeking precious metals will check nearby streams for mirrorfin nurseries, as large numbers of them are a sure sign of gold in the water.
Spoiler
Mirrorfins are fiercely territorial, especially the males, who have been known to kill each other over prime nesting areas. Females are more tolerant, and may even share a territory for a time to defend eggs or young hatchlings. They are larger than the males, and a few working together can make an illusory school big enough to frighten or distract much larger predators. The males prefer to fight challengers or threats with their bright, sharp horns, and mostly save their magic for elaborate dances to attract potential mates. They usually coat their whole bodies with gold, silver, and platinum, both as additional glittering attraction and to fuel their mating dance. Females keep their markings mostly to subtle flecks, more interested in creating a large number of doppelgangers than in keeping them all moving in ornate patterns.
Spoiler
Legend says the a gilded mirrorfin will bring great wealth to its owner, but the few breeders who specialize in these exotic fish would argue the opposite. Their bright horns and flecked scales are actually coated in precious metals, which they must consume to grow and thrive. It also seems to fuel their illusion magic, as their gilding quickly fades when it is used. But the illusions are dazzling while they last because the mirrorfin creates illusions of itself, turning these usually solitary fish into a glittering school.