Don't use the algae stuff, toss it. These products say "safe for fish" but for some reason a great number of fish get sick or die after using it. Your tank should have a complete water change , 100%, every other day. Fish hanging at the top is a sign of stress. Usually brought on by bad water parameters. Use a quality betta pellet and feed him 1 at a time. If when he doesn't eat one stop feeding and remove it ASAP. Some people use a turkey baster. Fish excrete ammonia, rotting fish food also adds ammonia as does fish poop. By the end of the week your betta is swimming in very toxic water. The problem with using a light to warm the tank is it can get too hot when on and cool down too much when off. Fluctuating temps is also a stressor. Check out this link. I would suggest a larger, heated aquarium that is cycled (or cycling) about 5 gallons, 10 is better if you have the means.

Here is a link that might be useful: Beta info

Thanks for the advice, I will change the water again and hope that saves him, then I will look into getting a bigger tank. It's weird he had been fine up until now, I guess that algea stuff is really bad.

As a side note about your algae problem, an alternative method for cleaning it would be to move the tank to a place with less sunlight (if it's getting any now). You could also get a little snail, whether you move it or not. Be sure to ask a lot about the type of snail though, since some get very big.

For algae problems: Brown slime algae (diatoms) is common in new aquariums usually after 6 months it goes away, silicates leach out of the glass or plastic and it is food for the diatoms. Bright light is supposed to help somewhat. I have gotten it slightly even with high power lighting. Once the silicates dilute the diatoms go away. Green water is usually too much phosphate in your water. It could be another nutrient but more often then not phosphates are the culprit. Phosphate comes from water supply as well as some low grade activated carbon and it is an ingredient in fish food because it is a nurient for them also. If uneaten food sits it not only creates ammonia but phosphates dilute. Try to keep your tank away from direct sunlight and only use your light for less then8 hours unless you keep live plants. Plants will compete for the nutrients and lessen them (phosphate is barely used by aquatic plants compared to other nutrients)and the plants will require up to 12 hours of light (preferably fluorescent). Nitrate will always be present (in a cycled tank) because it is the end product of the nitrogen cycle. So, keep lights minimal and algae can be avoided. My feeling is get a few live plants, even if they are just low light hardy plants, the Java Fern is perfect this way you can enjoy the tank with lights on for longer. I don't like going through the trouble of keeping fish to stare at a dark tank. The betta will love the plants and they are more natural then plastic ones. Not to mention they increase oxygen levels and help remove toxins.