1fish2fish is correct. Unfortunately just because pet stores can keep bettas alive in small containers for a few weeks does not make it humane to keep them in such small spaces for a variety of reasons. This is a very common misconception that has been reinforced by the way companies market products to consumers. The fact of the matter is, companies make more money off of tiny containers because they cost so little to make, they can easily jack up the price to increase their profit. People buy them because they don't like the idea that a $5 betta needs a $50 setup. That doesn't make it right, unfortunately.
Bettas come from vast rice paddies in tropical Thailand, and being cold blooded, they need stable temperatures of 78-83 degrees to be comfortable, healthy, and active. Cold temperatures cause weakened immune systems, and poor circulation--their entire metabolism slows down making it hard to digest. Food gets trapped in the gut and causes constipation. This is why you need a good heater with an adjustable temperature dial, not those cheap heater pad things.
Smaller tanks also cause other behavioral and health problems. As a consequence of being confined, many bettas develop neutrotic behaviors such as glass surfing and tail biting, many also become obese due to lack of space to exercise. Obesity is a leading cause of death in bettas. Healthy bettas kept in good conditions can live for 3-5 years on average, many get them to live to be 7 years old, one of the oldest bettas reported in captivity died at 15, if I recall correctly.
Small containers are very difficult to maintain, even though the water looks clean, fish constantly excrete ammonia through their gills--kind of like urine. This urine quickly builds up in small containers, to the point that it can actually burn the fish. Imagine constantly having to drink in, breathe in, and swim in pee so strong that it burned you. In order to keep such a small container clean, you need to do 100% water changes every 3 days, during which you should scrub everything out with hot water. You should have a thermometer so that you can match the temperature of the old and new water, and keep in mind that your fish should be slowly acclimated, even if the temperatures match.
If you are low on money but still want to give your fish an appropriately sized tank, I suggest getting a 2-4 gallon rubbermaid/sterilite plastic storage bin. They are safe to use with heaters, and are light and durable for frequent water changes. They are an ideal shape for housing bettas because they are long and shallow. They are also extremely cheap, only $2-$4 and make excellent quarantine and hospital containers as well.
Last edited by Adastra; 07-02-2010 at 04:45 PM.