Well im no expert but is there any physical damage to the tetras? maybe the betta is attacking them, also i found some water requriments according to wikipedia,
check your temp i think there a colder water fish
While commercially bred neon tetras have adapted well to a wide range of water conditions, in the wild they inhabit very soft, acidic waters that are usually cooler than the 25°C most tropical aquaria are maintained at.
[1] Neon Tetras have a lifespan of about five years.
Neon tetras are considered easy to keep in a community aquarium that is at least 60 cm (24 inches), with a pH of 5.0–7.0 and KH of 1.0–2.0. However, they will not tolerate dramatic changes to their environment. They tend to be timid and, because of their small size, should not be kept with large or aggressive fish who may bully or simply eat them. Fish that mix well in an aquarium are other types of tetras, such as the
rummy-nose tetra,
cardinal tetra, and
glowlight tetra, and other community fish that live well in an ideal Tetra water condition. They can also be with bettas (siamese fighting fish) including the male. Mid-level feeders, they are best kept in schools of five to eight or more, for the "
shoaling" effect when they move around the tank. They shoal naturally in the wild and are thus happier, more brightly colored, and more active when kept as a shoal as opposed to singly. Their colour and the iridescent stripe may become dim at night, and can be virtually invisible after a period of darkness. The color may also fade during a period of stress, such as human intervention into the tank. Neons are best kept in a densely planted tank with subdued light and an ideal temperature of 78-80 degrees Fahrenheit to resemble their native Amazon environment.
