Hi Adrena. The reasons the fish farmer gave you are certainly good ones, but there are a few more. Some fish do like to go exploring, so a polyethylene netting is a great choice to suspend tightly over your tank. Even the ones that jump out and are lucky enough to be scooped up and put back in, don't tell their friends that its really bad out there and don't jump out! But on a serious note, a shade cover will also inhibit algae growth in your fish tank. You will want to minimize algae growth for several reasons such as not having it take up valuable nutrients and that it has a very short life span and will go aneroebic in your system giving off ammonia and stealing valuable oxygen for your fish and plants.