Are those tetras? If so, it's pretty impressive they can get along with a betta. Beautiful crowntail by the way!
Hi they are White Clouds.
Keep in mind White Clouds are coldwater fish and their ideal temperature range has very little overlap with Bettas. What temperature is your tank running at?
As DW mentioned; WCMMs are coldwater fish, or Sub-Tropical at best. They need much cooler water than that of your Betta.