Betta lovers will often use a 'Betta condo' to allow them to keep multiple males in a single aquarium. The condo is simply a small container that is vented to allow water circulation through it. It hangs inside the aquarium, effectively keeping any fish inside it separated from the rest of the tank.
A certain degree of controversy surrounds the use of Betta condos. Single condos kept in separate locations within the tank are acceptable. However, when several are kept side by side it can induce males to flare at one another. Some owners, including myself, feel that the stress caused by this practice can negatively affect the health of the fish.
Does Tank Size Matter?
Recently Natalie, a Betta owner, posed an excellent question about tank size.
"I was reading the information that you listed about putting male Bettas in the same tank and that you can only put one in a tank at a time. I have found that this can depend on the size of the tank.
I have a 39 gal tank and on several occasions I have had two males and a female in the same tank. They did seem to chase each other around a little for the first few days, but once they were in the tank after that, they didn't give each other much mind. I have not tried this in a smaller tank so I don't know if it is just because they had enough room that it didn't matter or what.
Since that time I have often wondered if this is a myth or what, Because I am sure that there is more than one male in the rice pond at a time where they original from and they don't kill off all but one. " - Natalie
Natalie's observation is a good one. In nature more than one male lives in a rice paddy, however rice paddies are quite large, often encompassing miles of space. This allows each male to have his own territory. In small tanks there simply isn't enough room for the establishment of territories, so its not advisable to keep more than one male.
My observation has been that somewhere around 20 gallons is the point at which size becomes a significant issue. Sizes smaller than that are usually a problem. Sizes larger than that provide enough ‘territorial space’ to allow multiple males. However, few people keep Bettas in larger tanks. Traditionally Bettas are kept in very small tanks, hence the common statement of “one male per tank”. A more accurate way of putting it would be one male per territory.