I tell ya - I am CONFUSED as to what is the best water change routine!!!
Personally, I do one 50% w/c with gravel vac a week on my filtered, heated, cycled betta tanks (3 gals., 5 gals. & 10 gals.) All tanks have java moss, java fern & a moss ball in them.
I read on here two or MORE 50% or 100% changes are needed.
THEN, at Betty Splendens, they have THIS to say:
Make sure your betta is kept clean. A betta stirring up his own feces as he swims is just disgusting. Half-gallon containers will have to be changed twice a week. Gallon containers can go as far as once a week, and the larger the container, the longer the interval between changings. If the betta is kept in a 5+ gallon container with adequate filtration, normal tank maintenance of once a month or so is fine. Of course, if you see an accumulation of uneaten food, you should change his water sooner.
Be warned, there can be too much of a good thing, and changing the water too much can weaken your fish, forcing him to have to adapt and re-adapt to his conditions.
I have a couple boys pushing 2 years of age doing the once a week thing, but now I wonder if I'm doing things correctly!!!