A Betta fish is probably the most ideal fish for a bowl or small aquarium available. They are simple to keep, beautiful, and interact with the owner (they make great pets). Bettas also take up little space and are inexpensive to keep.
Maintaining your fish bowl can be difficult sometimes. However, your fish bowl should be kept as hygienic as possible. Doing so should help prevent any health care problems for your fish. When you are cleaning your fish bowl, do it as quickly as possible, because it is dangerous for your fish to be out of its fish bowl. Only clean your fish bowl with a cleaner that is safe for your fish.
When buying any pet, planning is necessary. Fish are not the exception to this rule. Their habitat might be very self-contained, but it is to be their home nonetheless. Temperature, space and environment are all key factors in ensuring a long life. For your fish's sake buy a book or use the internet to look into some of the basics of simple cold water fish care.
Water is a prime example. To many of us water's water, right? Well, no. You wouldn't plunge a new baby into a freezing cold bath and you shouldn't do the same to a fish either. If your goldfish comes in a plastic bag, float it in the bowl for some time before opening it, so that the bag temperature and the water temperature closely match. And that's before we even get on to acidity and alkalinity: pet shops again can provide solutions to test for these, as waste products and plants can change the nature of the water as, once again, changes aren't good for your fish's health.
Then there's the bowl. Fish bowls are the single most important factor in your fishes happiness and health. No other decision you make as a fish owner will be as important as what fish bowl you get for your fish. Many new buyers purchase the smallest round bowls, with little thought of how many fish they will be containing. Fish need their space too. Any reasonable pet shop will tell you the capacity for a size of fish in each bowl or tank and normally it will surprise you. Don't be conned by the old five second memory myth: as a child I had a fish that miraculously survived and swam for years around a murky round bowl. My parents eventually relented and bought a tank. My poor Goldie (original naming) couldn't cope - it kept hitting into the glass as it no longer had any idea how to turn round on its own. It died within days, unable to even properly raise itself to the surface for food. These fish develop habits that they can't change five seconds later.
I recommend at least a ½ gallon (2 liter) bowl for Bettas, although plenty of aquarists are successful with smaller bowls. Your large pea sized gravel or glass marbles for ease of cleaning. A small plastic plant or even a sprig of live anacharis makes a good plant decoration (make sure there is lighting overhead or the bowl is near a window for the plant to thrive). A ceramic ornament is nice too.
Plants, rocks, bits of wood: all of these provide the owner with additional things to look at and your fish with a more pleasurable environment to live in. All cost very little. Wash them and your bowl or tank regularly, especially if you haven't got a filter to do it for you, or an air pump to oxygenate the water. This will help stop the spread of illnesses and diseases, treatments for most of which can also be bought at most pet shops.
Sponge filters excel as small tank or bowl filters, for a betta, there is no better filter in my opinion. They are excellent bio filters and good mechanical filters in small aquaria. As for under gravel filters, there are small ones made for bowls or small tanks, but they take more maintenance than sponge filters (debris tends to collect under the plate). UGFs are more difficult to clean in small aquaria, as they are best maintained by vacuuming, which is difficult in bowls or small tanks.
A temperature between 70 and 80 F is best. For a bowl this is not really possible, just try and keep your bowl in a warm room of your house. If this is not possible, here are a few suggestions: An infrared reptile heat lamp works great placed in a desk lamp near your betta with a small thermometer in the bowl to know how to distance your heat lamp.
Change water at least weekly. In a bowl, I gently dip the betta out of the bowl with some water, then swirl the bowl marbles or gravel around and dispose of most of the remaining water. Then de-chlorinate the water with Start Right or a similar product and make sure the water is the same temperature as the water containing your betta. After this, add your betta back slowly and gently, including some of the water your fish is in.