Here you will find some things to consider before buying a fish bowl as well as information on how to setup your fish bowl.
Bowl. The first important thing to consider is the size of the bowl. 1.5 to 2 gallons is best. If you do not have a lot of space, you can get at least 1/2 gallon. Anything less than that is not recommended. While it is true that most pet shop owners keep Bettas in very small containers such as plastic cups, this is a very cruel thing to do! We all want a happily living fish, not a fish that merely survives. So please buy him a decent sized bowl. Not only would it provide a wider space for the fish to swim around, it would also hold a larger volume of water to dissipate waste and, therefore, require easier maintenance.
Appropriate Home. It is best to situate the bowl where the temperature is 25°C (77°F), but the fish will do just fine in a wider temperature range of 24°C (75°F) to 30°C (86°F). Since you cannot equip a small bowl with water heater, keep it away from sources of hot or cold air such as direct sunlight, air-conditioners, heater vents, etc.
Decorations. You can have a plant (live or plastic) in the bowl. They do not only make the bowl look nicer, but they also provide hiding places for the Betta. Live plants also reduce some wastes on the water. Watch for rotting leaves, however. They need to be cut as they will pollute the water. Rocks can be added as decor, and a broken clay pot provides interest to the fish. You can add gravel only after you learn to feed properly.
Equipments. Small, gentle filters are an option to help keep the water crystal clear, reduce waste, and therefore lessen maintenance. There are filters small enough to fit in a bowl, such as corner filters and undergravel filters. If you use one of these filters, it will also require an air pump to operate. Use an air clamp or valve to regulate air flow as Bettas do not like too much water movement.
Water. Once all the decorations and equipments are set-up, you can now add water. Fill bowl with bottled drinking water. TAP WATER IS NOT SAFE FOR FISH. If you are going to use tap water, treat it with chlorine neutralizer. If you live in the US your water probably has chloramine, so use a water conditioner that would neutralize both chlorine and chloramine. It would be best to use a water conditioner that will also eliminate ammonia and heavy metals such as copper and zinc, however this is not an absolute requirement. Add a pinch of aquarium salt or rock salt (not iodized salt), it is good for the fish. Let water stand for at least one hour before adding fish to let it warm up or cool down to room temperature. Take note that Bettas jump, so fill the bowl only at least two inches from the top. Adding a cover to the bowl is advisable, but make sure that air can freely move in and out of the cover as the Betta fish need to breathe surface air. Having a top on your bowl also reduces water loss due to evaporation and reduces the amount of debris that collects in the water from the environment.
Plan Ahead. Set aside water now for next cleaning. Fill a water bottle with tap water, leaving two inches of space from the top. Add a few drops of water conditioner and store covered for three days. This water will not only be of similar temperature but also chlorine free. If you are using bottled drinking water, it may be stored covered in its original container.
Transferring Your Fish. Now that your fish bowl is properly set-up, you are probably very excited to see your fish swimming around in his new home. WAIT, do not put the fish straight into the bowl just yet. Note that fish are extremely sensitive to fluctuating water conditions. The water from the store where your fish came from would have a different pH, hardness, etc. from the water in your home. Putting him in your new water without allowing him to "adjust" could stress him and cause him to get sick and even die. So, take time to do these steps:
Get a separate container, such as a big drinking glass and put the fish together with some of the water from the bag. Fill it by only about 1/2 of its capacity, as you will be adding some new water later. Let it stand for about 30 minutes or until the temperature of the water becomes similar to that of the water in your bowl. Use your fingers to compare the temperatures of the water from the store and your new water.
After the temperature has stabilized, you are ready for the next step. Scoop an equal amount of water from the bowl into the container. There should now be about twice as much water in the container. Let it stand for another fifteen to twenty minutes as the fish adjusts to the sudden change in water condition. You will probably need to cover the container to keep the fish from jumping as the water level is now higher.
Now your fish has adjusted to your new water and you are ready to transfer him to the bowl. Do NOT pour the water from the pet shop into the bowl. Using a small fish net, carefully net the fish out of the container and place him gently into the bowl.
Now your new Betta fish bowl is fully set.