Place the fish carefully in another container with half of the old water and the other half fresh water. Unplug any electrical equipment. Drain the tank. Separate 2 cups of dirty gravel and put aside. Place remaining gravel in a bucket and run forceful water over it, emptying it carefully when water covers the gravel. Do this until the water runs clear. Place the gravel back in the clean tank mixing the 2 cups of reserved gravel with the clean so as not to shock the fish.
