The marble gene is a fascinatingly frustrating gene. It can affect Bettas of all different colors and vary in its intensity. What’s even more intriguing is the rate at which some Bettas marble over others. While some Bettas take years to gain their full color, other marbled Bettas can seemingly change overnight. Sometimes you will buy a Betta and they will marble while they’re being shipped! There is even such thing as reverse-marbling where the Betta will lose their color almost entirely! Not only is the marble gene extremely volatile, but the phenotypes can seemingly “skip” an entire generation or two of offspring. I also worth noting that Koi Bettas are nothing more than marbled Bettas that have been line bred for certain Koi-like patterns. However, even Koi Bettas are susceptible to becoming solid-colored shadows of their former selves.
Here are some instances of Koi Bettas:
Other sources to consider: