Well, I'm not sure how often you would change it, I guess it would depend on your needs, just as long as your water quality is good you shouldn't have to change that frequently, but when my tank is up and running I'll probably change every week or bi-weekly, and you should do a 20-25% water change at a time, at least that's my understanding, if I'm wrong, someone will correct me and give you the 100% correct answer