You can download and install Animated Fish Desktop Wallpaper, and there will be fish swimming on your desktop.
This wallpaper is very simple, it only shows one fish moving in water. The fish is small, it wiggles its tail and moves across the entire desktop area; the water is blue and green, but it's static - which saves the system memory and CPU power. The fish moves slowly in vertical diagonal lines, it can't move horizontally or circle around. There are no other marine creatures or plants. When you open any program or any page, you will not be able to see the moving fish anywhere except for the taskbar.