I've seen a lot of threads on sororities asking pretty basic questions and although I know that there is a thread already dedicated to this, it seems kind of disorganised and the OP doesn't cover as many areas as I think it needs to.
I had this floating around on an unfinished site of mine so thought it might be helpful to those thinking of starting up a sorority tank.
Feel free to add any of your own experiences to this post. I think I covered most of the major areas where people run into trouble but if you think something is important post it up.
Introduction to Betta Sororities
Your first question might be, ‘What exactly is a betta sorority?’ In short, a ‘sorority’is a group of female Betta splendens housed together in the same tank.
A sorority is usually one of two things: a brilliant success, or a spectacular failure. There is more to a successful sorority than just putting a group of females in a tank together and expecting them to get along.
Below, I have outlined a list of steps that should hopefully help you on your way and tilt the odds of success in your favour.
Any sorority tank should be a minimum of 10 gallons
Female bettas can be as aggressive and unpredictable as males. Domesticated splendens have been selectively bred for generations, and were never really intended to live peacefully with others of their species.
Therefore, it is important to provide each female with enough space to get away from an aggressor, and to lessen the event of territorial flare-ups.
I usually ensure each of my females has at least 1 gallon of space per fish. However, you may want to increase this minimum if you have particularly big or hostile females. Unless your females are very young and it is only a temporary solution, I would never recommend a sorority tank be anything less than 10 gallons.
Finally, an ideal sorority tank is one that is wider rather than taller. You will often find the more dominant females will spend most of their time near the surface and may bully the others when they come up to breathe or feed. Therefore, it is best to give your females enough room to spread out.
You should provide as much cover as possible
By blocking the line of sight from one end of the tank to the other, you can drastically lessen aggression and prevent chases from happening.
In a sparsely planted tank, there is nothing to prevent one female from chasing another around for as long as she wants. As you can imagine, this is obviously stressful for the fish being chased, and usually once the fish is caught, there is nowhere for her to escape from an attack.
Ideally I like to see most of the tank covered with either artificial or live plants. You want plants that provide cover at all levels of the tank. It is no use having lots of very short plants at the bottom, when your females are chasing and fighting up top!
I found my females rarely if ever went into hides such as terracotta caves or PVC pipe tunnels. Other people may have different experiences, but I have found the most favoured hiding places were in amongst the plants, particularly up near the surface.
You need a minimum of four-five females
A successful sorority is often one that is over rather than understocked. Indeed, you will find it very difficult to maintain long-term a sorority that has only 2-3 individuals. Generally the most dominant individual(s) will attack the weakest until she is either dead or extremely stressed.
To avoid this from happening, you want to have enough females that any aggression is dispersed rather than targeted at the weakest female. Some people are much more conservative with the stocking of their sororities, but I feel that the more females you have, the less prolonged aggression you will see.
If you cannot purchase or house any more than 2-3 females, I would advise against a sorority. It is only be a matter of time before you see the results of poor stocking, in the form of disease, injury and even death. An alternative solution in these instances is to either divide up a single tank, or house each female separately from the other.
It is best to choose the youngest females possible
Older, mature females that have lived alone are more inclined to react with extreme aggression when placed in a sorority environment. While most tend to adapt and become part of the community after an initial introduction period, some will continue to act aggressively and will need to be at removed.
Younger females (siblings from the same spawn are often the best candidates for sororities particularly if they have yet to be separated) are usually much less territorial and aggressive. There is usually much less serious fighting between individuals while sorting out a hierarchy, and they tend to be much more flexible with the introduction of new tankmates.
However, one danger in choosing young females is that you will end up with a male plakat instead. This has happened to me once or twice, and for those without separate accommodations, can be quite the headache.
Females tend to have a rounder body shape than males, and in light coloured females, you should also be able to make out the ovaries as a small yellowish triangle behind the intestines. Male plakats will often have longer ventrals, a pronounced beard, possibly a fuller caudal, and generally a sharper angle to their anal fin. While they may not have a visible eggspot, it is not uncommon for younger males to have an ovipositor as well . Therefore, the presence of an eggspot alone is often not a good determinant of gender.
The best advice I can give, is that if in doubt, don’t purchase.
Ensure that your water parameters are nothing short of perfect
Living in even the most peaceful sorority environment is often quite stressful for a female betta. She must not only compete for food and for territory, but also must deal with the close proximity of equally territorial and aggressive fish.
Stress in fish is one of the number one causes for outbreaks of disease. While poor water quality may not kill your females outright, it can compromise their immune system, leaving them vulnerable to diseases a healthy fish may otherwise have managed to fight off.
In poor water conditions, when one female becomes sick it is usually quickly spread to every other female in the tank as well. This is why it is important to monitor your water parameters and ensure that they are always within an appropriate range. Any readings less than ideal demand an immediate water change and a look at the way the tank is being maintained.
Aggression in sororities
The question I often hear from most of those new to sororities is "What is too much aggression and when should I intervene?"
You have to remember when dealing with bettas, that aggression is a perfectly natural behaviour. However, this does not mean that your sorority tank should look like a WWF wrestling match.
It is common for there to be some fighting when you first introduce your females. This could be anything from posturing, body slapping, flaring, chasing and biting. This usually settles down within a couple of days although it is perfectly normal to see occasional flare-ups.
For me, the time to intervene is when there is more than a couple of minutes of sustained fighting between two females in which obvious damage is being done. I separate out whoever looks to be the aggressor, and put them into a breeders' net that sits in my sorority tank. I find this is often the best method of introducing particularly aggressive females as by the end of the week, they are generally accustomed to the other females being nearby and less inclined to over react when reintroduced.
I also intervene if it is obvious one female is being continually bullied by the others. If a female is starting to lose a lot of condition, or has severe fin and scale damage it is time to pull her from the sorority. It can be difficult to understand why certain females are targeted by the others. Just because a female is small or less aggressive does not mean she will be the one bullied. Therefore, it is important with a sorority tank that you find the time each day to check on your females and see that everyone is in good physical shape. I speak from experience when I say it does not take long for things to suddenly fall apart.
Sorry if there are any spelling mistakes or issues. I went through it in Word but may have missed some of the less obvious grammatical errors.