The halfmoon betta is one of the most prized betta varieties in the world. Named for its spectacular tail that spreads a full 180 degrees — forming a perfect half-moon shape — this variety is the result of decades of selective breeding by Thai betta breeders, including those in Uttaradit, one of Thailand’s premier betta fish breeding regions.
This guide covers everything about the halfmoon betta: what defines the variety, the most popular colour forms including the royal blue rosetail halfmoon, care requirements, and what to look for when buying one.
What Is a Halfmoon Betta?
A halfmoon betta (Betta splendens) is defined by its caudal (tail) fin, which spreads at least 180 degrees when fully flared. The tail edges are straight and the spread is symmetrical — the dorsal and anal fins also flare prominently, creating a dramatic fan-like silhouette. The halfmoon form is judged harshly at competitions: any tail that falls short of 180 degrees, shows ray branching (rosetail), or has irregular edges is disqualified from the highest grades.
Royal Blue Rosetail Halfmoon — The Showpiece Variety
The royal blue rosetail halfmoon is a highly sought-after variant where extensive ray branching creates ruffled, rose-petal-like edges on the tail. Rosetail bettas are visually stunning — the extra branching creates a flowing, layered tail that is unlike any other variety — but this trait comes with trade-offs.
Because of the excessive fin tissue, rosetail bettas can struggle to swim efficiently. The fins may fold or clump in lower water flow. They are best kept in calm, filtered water with excellent water quality to prevent fin rot, which rosetails are more susceptible to than standard halfmoons. Despite these care requirements, the royal blue rosetail halfmoon remains one of the most photographed and collected betta varieties worldwide.
Popular Halfmoon Betta Colour Varieties
- Royal Blue — deep, metallic blue across the entire body and fins; the classic halfmoon colour
- Red — vivid solid red; among the most competitive show colours
- White Platinum — pure white or cream; striking against dark substrates
- Black Orchid — dark body with iridescent blue-black fins; very popular in Thailand
- Mustard Gas — yellow-green body with blue or purple fins; a bi-colour classic
- Koi — orange, white, and black marbling; highly variable and collectible
- Galaxy — multicolour with metallic speckled pattern resembling a galaxy
Halfmoon Betta Care Requirements
Halfmoon bettas have the same fundamental care needs as all bettas, but their large fins require extra attention:
- Tank size: 10 gallons minimum — larger tanks allow more swimming room for the big fins
- Water temperature: 78–82°F (25–28°C) — consistency is critical; fluctuations trigger fin rot
- Filtration: Sponge filter or baffle a standard filter — strong flow tears rosetail fins
- Water changes: 25–30% weekly; pristine water prevents fin rot and bacterial infection
- Tank décor: Avoid sharp plastic plants or ornaments that snag fins; use silk plants or live plants
- Companions: None — halfmoon males must be kept alone; they will fight any other male betta
Halfmoon Betta vs Plakat Betta
The plakat is the short-tailed form of Betta splendens — the body type closest to the wild betta. While halfmoon bettas are bred for spectacular long fins, plakat bettas have short, strong fins that make them faster swimmers and more active. Plakats are also more resistant to fin rot and generally more robust. Both are bred extensively in Thailand, and many serious Thai breeders work with both varieties.
Buying a Halfmoon Betta — What to Look For
When selecting a halfmoon betta, look for:
- Full 180-degree tail spread when flared (view from the front and side)
- Symmetrical finnage — dorsal, anal, and caudal fins all match in height
- No fin holes, tears, or black edges (signs of fin rot)
- Active swimming, alert posture, and vivid colour response when you approach
- Clear eyes — no cloudiness or popping
- Rounded belly — not sunken (malnourished) or bloated
Thai-bred halfmoon bettas from reputable breeders in regions like Uttaradit tend to be of exceptional quality. Many are available through online betta marketplaces and ship internationally in insulated bags with oxygen.