How often need to change water?

Hi everyone i just started rearing freshwater fishes in a 4ft tank for aroudn a month now~

How often do i need to change the water?

The water in my tank turns yellowish/brownish already after around 1 week, which i suspected is due to the log i put in~

My filter is the on top of the fish tank type, the long black rectangular kind~
The sponge turns extremely brownish also after 4days~

Not sure if frequently changing the water will harm my fishes, 1 week once is it counted as frequent?