"freshwater fish are in a hypotonic environment. how do their gill chloride cells function in this environment? "

Freshwater fish are in hypotonic water (lower concentration of solutes) environment and therefore, water is continually diffusing into the fish through the gill membranes. The gills membranes are also permeable to respiratory gases, ammonia waste products, and ions (sodium and chloride). All together water moves in towards the higher osmotic pressure of the blood, sodium and chloride ions diffuse out of the fish (down their concentration gradients) to the external environment. These processes expend energy.