Furuno CSH-7 - Manual (Page 14)

1-7
Adjusting the Gain
The GAIN control adjusts receiver sensitivity (gain). Adjust it
so fish echoes are clearly displayed with minimal noise on the
screen. Too high a setting not only displays excess noise and
makes it difficult to discriminate wanted echoes but also causes
seabed echoes to be painted in strong colors, resulting in echoes
being masked by seabed reflections. Normally, set the control
somewhere between positions “3” and “7”.
Gain low Gain proper Gain high
Figure 1-6 Gain settings and resulting picture