A good rule is to change 10 to 15 percent of the water each week. If your tank is heavily stocked, bump that up to 20 percent each week. A lightly stocked aquarium can maybe get by for two to four weeks, but this should be the maximum length of time between water changes.
The Pythagorean theorem is the square of the length hypotenuse side of a right triangle equals the sum of the squares of the lengths of the other 2 sides. <span />
