1fish2fish is correct, with a tank this size you should be changing the water every day. Fish constantly excrete ammonia through their gills, kind of like urine, and this builds up to the point that it actually burns the fish. Think about how uncomfortable it must be to have to swim in, breathe in, and drink in pee all at once--not pleasant. Dechlorinators can turn some of this ammonia into less harmful ammonium, but that's not going to cut the mustard on a tank of this size with a higher ammonia concentration. What that is intended to do is help neutralize the ammonia already found in some people's tap water in very small amounts.
It's very common for people to be mislead by how products for bettas are marketed in pet stores. Simply because you see a box with a happy betta on it does not mean that the "tank" was designed with a betta's well-being in mind. The truth is that these tiny tanks take very little materials to make, so they can jack up the price and make a higher profit on little "tanks" than they can on some larger ones. They don't care if your fish dies a terrible death, the company just wants your money.
With that in mind, also consider that every creature needs suitable heat, proper sanitation, exercise, environmental enrichment, and should be given the ability to express their natural behaviors. That isn't possible in a tank this small. Even if you could maintain pristine water quality in this container, it still won't be good enough.
I suggest returning your betta tank and getting something like this: http://www.fosterandsmithaquatics.co...48&pcatid=3848
this tank costs a bit more, but it's very well designed, it comes with a good fluorescent light, a filter, it's large enough to be heated, and it's large enough to maintain a cycle which will make it so much easier for you to maintain. It takes up very little space but gives your fish enough room to be happy, healthy, and show off for you. It's also acrylic, which makes it light and extremely durable. I've dropped one of these on my kitchen floor and it was fine. If cared for properly, most bettas live for 3-5 years, some live for 7, and one of the oldest died at 15. This pet is going to be a major facet of your life for a few years, you might as well invest a little more into its housing--it will make your life so much easier.