There is no right and wrong, too much or not enough. Every tank is different, and every aquarist has a different routine. How often you do the water changes makes a difference too (8 gallons per day is probably more then necessary, 8 gallons per month is probably not enough). A typical overstocked mbuna tank will do well on 25% every week. Use this only as a starting guide, you can create your own routine according to your own aquarium and schedule - there are no rules.