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What is a Halfmoon Siamese Fighting Fish (Betta)?

Updated: Jun 7, 2020


I’m sure many of you have seen these magnificent Bettas online and thought how great would it be to own one. One search online would give you hundreds, if not thousands of sales pages selling Halfmoon Bettas. We know it’s tempting to just buy it all but the first question you should be asking yourself is, are those Bettas you’re seeing qualify as Halfmoons?

What exactly is a Halfmoon? As the name suggests, a Halfmoon’s caudal fin must have a spread of at least 180 degrees when the fish is in full flare, similar to one half of a full moon, hence the name Halfmoon.

A Halfmoon Betta with a caudal spread of 180 degrees. This is the most basic requirement of a Halfmoon.

Now some of you may ask, how about those Bettas with a caudal spread of less than 180 degrees? Those are what we call Super Deltas. Super Deltas, or SDs, are simply not as desirable as Halfmoons. These fishes should never be mistaken or wrongly labelled as Halfmoons, unless you have a big problem with simple geometry.

A Super Delta. Notice the spread of its caudal fin being lesser than 180 degrees.

What makes a good Halfmoon?

Alright, now that we have already confirmed whether the fish you’re looking at qualifies as a Halfmoon or not, let’s take a closer look and see the points which make a Halfmoon an awesome one.

Balance and Proportion Like almost everything on earth, something that is proportionate in looks will tend to be more appealing than an unbalanced one, and show Bettas are no exception. But wait, what do we mean by a balanced and proportionate fish?


It is ideal for the length of the caudal fin to be somewhat equal to the body length (as shown by the red lines). Same for the dorsal and anal fin; it shouldn’t be too long and should ideally be of the same length overall (as shown by the green lines). The base of the dorsal fin shouldn’t be too wide in Halfmoons and should ideally be about half of the body length. Basically, the main points to take note are: - Fins shouldn’t be too voluminous for its body to carry. - Body should not be too skinny or too bulky in relation to the overall volume of the fins. - Length of the fins should not be too long. It’s all about the balance and its overall look.

Common faults to take note We know, when most of us see something we like, we tend to just dive in and get it but often miss out on the finer points. Here are some pictures to help you identify the undesirable traits in a Halfmoon betta.

Excessive Branching

Notice how the fins overlap each other? We call this a fish with heavy ray branching, but some may know this as a Rosetail. Yes it may look pretty to the untrained eye, but this is definitely an undesirable trait as it ruins the overall balance of the fish with regards to its caudal fin. This is especially undesirable when you are going into breeding of show bettas as it increases the chances of the next generation showing the same rosetail trait, not to mention deformities that usually come along with such a trait like misaligned scales, underdeveloped bodies etc.

Unbalanced Fins

Here’s another fault that is commonly missed out by people who just started out in this hobby. Notice the exceptionally long anal fin? That is also one of the undesirable trait when looking out for in show bettas as it ruins the overall balance of the fish.

Short caudal fins and long dorsals will also create an unbalanced fish.

Ray Curling, Random Rays

These photos show another common, easily missed fault when choosing your elegant warrior from either online or at the stores. Notice the curled rays on the ventral and anal fins? These usually occur when the fish is kept in bad water condition for a period of time. It can also result from shipping. High ammonia in the bag can cause burning and curling.

As these curled rays are usually caused by external factors it is safe to get them just for the purpose of breeding.

So in conclusion… There are many other faults that you need to look out for when selecting a show Betta but the main purpose of this piece is to actually help all of you out there in identifying a true Halfmoon and the differences between a real Halfmoon and those Halfmoon wannabes Super Deltas out there.

Are there any other topics you wish to know more about? Contact us and indicate which topics you would like to read more about! However do note that we are neither Betta scientists nor the great old elusive gurus of Bettas so do not expect us to know every single thing under the sun about Bettas, but we will still try our best in answering all of them!

With thanks to Ezkiel Goh

Copyright 2019

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