If you use the Deeper sonar in a tight artificial water tank (pool, sink, bowl of water, etc.) - it may show fish icons when in reality there are no fishes there. If you see something like that, there is no need to worry.
What happens is that ultra-sound beam emitted by the Deeper gets reflected from the border of an artificial water tank, and the sonar is unable to process this ultra-sound properly. Thus, it gets confused and displays improper readings like "phantom" fishes or incorrect depth.
For any sonar to work properly, its ultra-sound beam (in a form of a cone) has to reflect the bottom and come back to the unit for processing. If there's a giant obstacle that interferes with the beam ( for example - border of a boat or pool) - sonar will definitely show incorrect data. This can happen with any kind of sonar system that is tested in a small artificial water tank.
You can learn more about how sonars work here.