You may have heard that betta fish should be kept alone. That’s a simple solution to a more complicated question!
Male betta fish will fight each other if they are too close together in an aquarium. So if you’ve got a small tank (like twenty gallons and under), it’s probably best to have just one male betta fish. Females are somewhat more tolerant of each other, and can share if there is adequate room in the tank.
In nature, there will be more than one male betta fish in a body of water — but that body of water can span miles of space! Each male in the wild has his own territory. In a tank environment, each male may not have the space to establish a territory. That’s why experts suggest keeping only one male betta fish in a tank (especially if your tank is twenty gallons or less). In larger tanks, you may be able to keep more than one male betta fish without the fish fighting.
Another option is using a “condo” to keep your fish separated in the same tank. A condo is a small container that is vented to allow the tank water to circulate in and out. A fish inside the condo can see the fish who are outside, and vice versa. Some aquarium keepers feel that this setup can be stressful to betta fish both inside and outside the condo. If you want to try keeping multiple male betta fish using condos, try keeping your condos at opposite ends of the tank, rather than right next to each other.
However, most people tend to keep betta fish in very small tanks — even flower vases and small jars! Some experts feel that this isn’t healthy for the fish. Although the betta fish comes from shallow rice paddies and swamps, that doesn’t mean they will get the same environment from a vase or jar with a few plants in it. Unfiltered environments (like vases and jars) can allow dangerous waste to build up, endangering the health of your fish. And without a heater, that tiny vase may be too cold for a fish that is used to water temperatures around eighty degrees Fahrenheit.
Consider this, too: in the wild, betta fish are primarily meat eaters (insects and larvae). The plants in that little jar are edible, but don’t provide the best food for your betta fish.