You should be getting "perfect" arches if your 520 is set up correctly. Not while running, you should see black dots/pixels and if they are large enough you should see red in the middle of those balck dots. And bigger fish you should see yellow inside the red. At speeds up to about 22 mph. Depending on how you have the transducer and unit setup. You could see that at higher speeds. But for trolling speeds, they should show as "perfect arches". Bruce has some great info and dvd's.
http://www.doctorsonar.com/