In a mature tank depending on size, a once a week 25% water change is good, but you need to test your water and see what your levels of ammonia, nitrites and nitrates are, get back to us with those and we can help you better, also please if you will let us know the size of your tank and your livestock
Since I've been adding fish I do about a 35- 40% water change and test the water weekly. Then I add fertilizers and what not because I have plants. I usually do about 30% on average.