First, you must understand that you should take everything which your local fish store tells you with a grain of salt generally. Also, you need to realize that the eventual by-product of your fish once your tank is cycled is nitrates. As a result, there is no harm in changing your aquarium water too often. The way I see it is like this. When you keep betta fish, you generally will not keep them in a very big tank, generally a small tank like 5 gallons. As a result, you can get like a 1 gallon bowl and then every week scoop up some water, dump it, refill and add prime and do a 20% weekly water change with like 2 minutes of effort.
The thing to realize is doing water changes once a week versus every other week will not hurt. In fact, it will probably help. So with a small aquarium, why not?
