Betta Fish Habitat
The minimum tank size appropriate for a betta fish is perhaps one of the most controversial topics in betta fish husbandry. People see betta fish in tiny, unheated bowls in stores and assume that means such an environment is perfectly suitable. You could probably keep a dog alive if you left it confined to a tiny, cold closet, but you could never expect it to be healthy or thrive. The same holds true for betta fish.
Betta Fish Swim in their Bathroom (The Importance of Tank Size)
It's true. They expel waste into the same water they swim in. And not just through their excrement; all fish constantly excrete a toxic substance called ammonia through their gills. The less water a betta fish has to live in, the higher the concentration in the water of ammonia and other harmful substances. Think of pouring a cup of salt into a cup of water vs. pouring a cup of salt into a gallon of water. The cup of water will have a much higher concentration of salt because there is less water for the salt to disperse in.
Fish keepers do water changes to help remove water containing harmful substances from their aquarium and replace it with new water to lower the overall concentration of these substances. An aquarium with more water for harmful substances to disperse in will have a lower concentration of harmful substances in the water and will therefore require water changes less frequently than a smaller aquarium.
Housing your betta fish in a novelty 1 - 2 gallon "tank" or smaller will result in harmful substances accumulating in the water quickly and require you to perform partial water changes every other day and a 100% water change once a week. That's a far cry from simply leaving your betta fish alone in a tiny bowl like the setup in many stores might suggest you can get away with. Without regular water changes, a betta fish in such a tiny enclosure will unfortunately succumb to harmful substances in the water quickly.
We recommend making things easier on yourself and your betta fish by treating it like, well, a fish, and housing it in a setup of at least 2.5 gallons, the more the better. In a larger tank, harmful substances will be less concentrated, requiring less frequent water changes. A larger tank will also provide your betta fish room to swim, allow room for decor that both you and your betta will appreciate, leave room for carefully chosen tank mates, and, perhaps most importantly, allow room for a heater.
Another benefit of a larger tank is that it can be cycled. "Cycled" refers to having accumulated enough beneficial bacteria to break down harmful substances in the water to much less harmful substances. You can think of the beneficial bacteria as doing part of the job of removing harmful substances for you. Specifically, the nitrogen cycle breaks down the ammonia a fish expels into nitrite, which is then broken down further into a less harmful substance called nitrate. When you do water changes in a cycled aquarium, you are primarily removing nitrate as opposed to ammonia, which is much more toxic. (See this link about how to cycle your aquarium)
The "Tropical" in "Tropical Fish" Doesn't Mean Exotic (The Importance of a Heater)
Betta fish are tropical fish. Many people think "tropical" means pretty, exotic, or some other quality that warrants an umbrella in a drink. However, "tropical" actually refers to a climate. Bettas come from tropical areas in Asia such as Thailand, Malaysia, Cambodia, etc.
As tropical fish, betta fish require water at a constant temperature within the range of 77 - 83 degrees Fahrenheit. Otherwise, they can become very lethargic. If the water gets too cold, they can even die. For that reason, heaters are considered very important for taking proper care of betta fish.
Even if your betta's water is often within the range of 77 - 83 degrees Fahrenheit without a heater, the weather/temperature can change rapidly, and typically drops considerably at night. Most* aquarium heaters account for changes in temperature, turning on when the aquarium water drops below the desired temperature and turning off when it isn't required to sustain it.
An aquarium heater will therefore ensure that the water in your aquarium stays a constant temperature, which your betta fish needs in order to thrive. It is for that reason that we recommend housing your betta in an enclosure at least large enough to reasonably accommodate a "mini" aquarium heater, which are typically specified to be for 2 - 5 gallon aquariums.
*NOTE: You should make sure that any heater you get has a thermostat that allows you to set the desired temperature and accounts for the actual temperature of the aquarium water. Otherwise, the heater may not make the water warm enough, may make it too warm, or may allow the temperature to fluctuate, none of which is good for your betta fish. Unfortunately, many mini heaters lack this basic feature. However, the smallest adjustable heaters are only a few dollars more than mini heaters, and are well worth it for your betta's health and your peace of mind.
Come on in, the Water's Fine (The Importance of a Thermometer)
Or is it? A thermometer is recommended to ensure that your heater is doing its job and your aquarium water is the right temperature. There are many small, inexpensive, paper thin thermometers available that can be stuck to the side of a tank to indicate its temperature.
[Don't] Take My Breath Away (The Importance of Access to the Water Surface)
Betta fish need air. That's right, air, not just oxygen. While most fish extract the oxygen they need from water via their gills, betta fish have a specialized organ called the labyrinth organ that allows them to take in oxygen directly from the air. Betta fish therefore need access to the water surface where they can breath, or else they will drown. When decorating your tank, keep in mind your betta's need to have easy access to the surface of the water.
Since your betta will be visiting the surface of the water from time to time to breath, ideally the room temperature will not be radically different from the temperature of the aquarium water. Keeping your tank covered via a hood or glass cover will help keep the air above the tank moist and warm for your betta fish, as well as slow down the rate of evaporation.
Keep It Clean (The Benefit of a Filter)
While not considered strictly necessary for betta fish, a filter can help keep aquarium water clean. Betta fish should have very little water movement in a tank, so if you opt for a filter for your tank, make sure it has a very low flow. You can ensure a low flow by either getting a filter rated for a smaller tank, "baffling" a filter to restrict its rate of flow, or both.
Nice Digs! (The Importance of Decorations and Plants)
Decorations and plants that offer hiding spots make betta fish feel secure and allow them to get away from light and rest when they want to. If your decoration has holes for your betta fish to swim through, make sure your thumb can fit through all of them to ensure that your betta fish won't get stuck. Similarly, make sure that your decorations and plants do not have very sharp edges, as betta fish have delicate fins. A good way to test whether your decorations or plants can harm your betta fish is to run a pair of nylons over them. If the nylons snag or tear, your betta's fins will, too.
Live plants are not necessary, but they are considered helpful in keeping water clean and giving betta fish natural places to hide and rest. Floating plants such as hornwort are highly recommended and are easy to maintain. Java moss is also very easy to care for and can be floated on corks. Java fern, while not a floating plant, also does not require much light or maintenance. All live plants do require some amount of light, however, whether artificial or from the sun via a window.
Don't Keep Me in the Dark (The Importance of Lighting)
In nature, betta fish live in rice paddies where they experience sunlight during the day and darkness at night. It is considered beneficial to emulate this natural experience in your aquarium. You can use artificial light or rely on light from the sun coming through a window if your aquarium is positioned to receive it.
If using an artificial light, make sure that it is not heating your aquarium water above the recommended temperature of 77 degrees Fahrenheit. Most light fixtures made for aquariums of a particular size are designed not to have much impact on water temperature. A simple desk or reading lamp is also a fine source of light so long as it is positioned far enough from your aquarium so that it doesn't raise its temperature.