There really is no exact percentage to be changed at an exact frequency. While all fish need stimulation to thrive, goldfish seem to especially need it. In a tank, to provide the fish with more excitement, about once a month, rearrange the fake plants and ornaments in the tank. Also, provide a variety of foods from flakes to pellets to live foods to frozen to freeze dried. Ponds over a few hundred gallons provide plenty of excitement with new places to explore, live food around every corner, and other fish.
Physical stimulation comes from having room to swim, other goldfish to touch, and a chance to spawn.
Myths About Goldfish 1. Goldfish can live happily in a bowl. Without aeration or filtration in bowls, the water will always be dirty and low in oxygen even with daily water changes. The goldfish will not stop growing to fit the size of the bowl as many people think. Goldfish are hard to breed.