If your bowl is one gallon or smaller, you should actually change the water MORE often than once a week. Pet store employees are often misinformed, or don't think that fish deserve the respect that any pet deserves. When you have a fish in a bowl, they are usually unfiltered. Ammonia builds up in the water and can injure the fish and make him more susceptable to other maladies. My betta is in a one gallon bowl and every three days I do a 50% water change, using a clean (never touched soap) turkey baster to remove half the water and replace it with dechlorinated water of the same temperature. If I see any poops on the gravel between water changes I use the turkey baster to get them out as well. Once a week I do a complete water change, wipe the bowl down with hot water and a paper towel. NEVER use soap or bleach on anything that will come into contact with the fish.
Mine was listless when I first got it because I was told to change the water once every week & a half to 2 weeks, and I didn't keep his bowl in a very warm room. With dirty water, bettas are more susceptable to parasites like ich or velvet, or an infection. Mine developed velvet after a while when I didn't clean his bowl often enough. Room temperature is often not warm enough for bettas, as they are tropical fish. If you can afford a slightly larger heated and filtered tank, you will find that bettas are much more lively and healthier.
I would clean your bettas water more often, once a week is the minimum if your bowl is a US gallon or less. If it is half gallon sized, many would recommend that you change the water completely every other day. Also, when my fish had velvet due to poor water, he also acted lethargic with clamped fins. I would check him carefully for little white spots (ich) and then shin a bright light on him and see if he's looks like he was sprinkled with copper flakes (velvet). Both are parasites.If you find that your fish does appear to have a parasitic infection, or does not improve with better water quality, I would post pictures and a description on a more frequently updated betta specific forum, such as
The Betta Bug. Personally, when I do a 100% water change once a week I add 1 drop of Aquarisol anti-parasitic to prevent parasites from appearing. Good luck with your little guy!
