There are 2 ways fish are shown on the Deeper App. The easiest way to detect them is to turn on Fish Icons. Here, the app interprets the sonar readings, identifying some of them as fish. Fish Icons have 3 sizes – small, medium and large. Correctly identifying a fish and its size depends on many variables, with the weight of fish just one of them. Fish Icons are not the most accurate way to judge a fish’s size, because the algorithm could display one bigger fish when actually there are actually a few small fish. The most accurate way to judge fish size is to turn off Fish Icons. With Fish Icons switched off, fish will then be shown as an arch. There is a learning curve to this way of finding fish, and in the beginning it can seem confusing. But with practice you will be able to determine what is a fish and how big it is.

Here you can find an in-depth guide for this way of detecting fishes:
Finding fish and underwater objects with PRO series

 

 

Lots of small fish shown with and without fish icons

 

Fish arches show a few small fish next to each other, but the small size is due to a low sensitivity setting. In reality, the fish was much bigger.

Carp shown on the app and in reality

 

A car underwater