Definite noticeable changes occur in koi and goldfish, plants and the pond itself as the temperature of the water begins to drop. Leaves on water plants start to brown and die back. Koi and goldfish become less active and require less food. Leaves fall and cover the surface of the pond. The pond may suddenly clear up after not being clear all year. These are changes we see. The changes we do not see are just as important. Nitrifying bacteria, present all spring and summer in the biofilter, stop reproducing and become none effective. Biological functions in the fish’s bodies, controlled by temperature of the water, slow down and eventually the fish hover at the bottom of the pond in a state of semi-hibernation.