His problem is likely ammonia poisoning. As was mentioned earlier, a tank that size is so small that it must be 100% changed every day in order to maintain proper water quality. Please understand that fish constantly excrete a toxic substance called ammonia through their gills--this is kind of like their form of urine. In nature, this toxic ammonia would be broken down into a less harmful substance, consumed by plants, and diluted by much larger quantities of water. Since none of these components are at work in your small tank, you have to compensate for that with frequent water changes.
You should also consider that bettas originate from a tropical climate where the temperature is consistently around 80 degrees. Since they are cold blooded animals, being subjected to even a few degrees below that can have a significant impact on the speed of their entire metabolism. The average room temperature (72-73 degrees) is really much too cold for a betta to thrive in. Being at these low temperatures causes problems with circulation, digestion, and a weakened immune system.
You should really consider upgrading to a larger tank. The recommended minimum size is two gallons, because this is the volume that most quality heaters are designed for. Keep in mind that the larger the tank, the less maintenance you will have to perform to keep your fish healthy and the more stable everything will be, so if you decide to upgrade; think big--5 gallons is a wonderful, stable, manageable size, and many come in compact designs that take up very little desk space.
If money is a problem for you, you can opt for the cheap "kritter keeper" style tank, or for even less money, you can get a 2 or 4 gallon rubbermaid/sterilite clear plastic storage bin from WalMart or Target for about $3. A heater should help him immensely, but keep in mind that when you purchase a heater, you get what you pay for. Cheap pre-set heaters, heater pads, and other non-adjustable style heaters don't actually work on a thermostat, so the temperature can fluctuate and they often overheat the water or don't get it nearly warm enough depending on the air temperature in your house. Don't waste your money on a non-adjustable heater. Instead, pay just a few dollars more for a 25 watt heater that has an adjustable temperature dial--it is worth the investment.
Once your betta is in consistently clean and warm water for a few days, he should perk up quite a bit. If you can find it, some methylene blue should help with his ammonia poisoning and prevent secondary infections. If you're thinking of making any purchases for your fish, please let us know so we can give you advice about whether or not the product is worth your money. Good luck with your fish.
Last edited by Adastra; 10-03-2010 at 11:18 PM.