That's true. You should always leave an amount of the old water in the aquarium so as not to shock the fish. Keeping a fish bowl clean is the same as keeping an aquarium clean although it probably needs change more often as a bowl is a smaller space and the water will become dirty more quickly. Using a clean container, pour 1/4 of the water from the bowl in it and using a net, place the fish in this water. You can use a water siphon or just pour out 3/4 of the remaining water in the bowl. To save the gravel, simply place something over the drain to catch any that falls. Then refill the bowl with fresh water and let it stand until it reaches room temperature. You are then ready to put the fish back in the bowl along with the water they were in. If the water becomes really dirty inbetween regular cleaning times, remove half and replace it with room temperature water.
