My experience with fish keeping has been short, so bear that in mind. However, mathematically, if you make only one change per week and are changing less than 100%, your nitrate level will continue to grow week after week until it hits a mathematical limit. It will then fluctuate between two points.
Easy to make a quick excel sheet telling you where that limit will be if you know two data points. First, how much does your nitrate level increase weekly. Second, what percentage of water do you change in your weekly water change.
For example, if your nitrate increase by 20ppm weekly, with a 33% water change, after about three months your nitrate levels will fluctuate between40 - 60 ppm. It will be 40ppm directly after the change and will increase to a high of 60ppm directly before the next change. With the same nitrate increase of 20 ppm weekly, a 75% weekly change will have your tank fluctuating between about 7ppm and 27ppm weekly after about three weeks. Meaning your tank should never hit 30.
This doesn't take into account things like plants that reduce nitrate levels, but the difference is pretty clear. A 75% change once a week is substantially more effective if your goal is maintaining consistently low nitrate levels.
