Air/gas bubbles do get FishID symbols. If you notice the line of ID symbols at the surface of the above screenshots, those are from air bubbles created by the hull. You get less of those if you don't turn on 83 kHz, but then you won't get both coverages.
When you change the FishID Sensitivity setting, you can reduce the size of the fish being detected or increase the size of the fish being detected within the parameters of the size ranges. A high setting will display all three symbols and a low setting will only display the largest symbol.
I have posted these before, but here it is again. I use FishID to see if there are fish within cover that I want to try to catch.
I scan this laydown each time I am at this lake. If I don't see any symbols, I don't stop to fish it. These screenshots are a few days apart. When I saw the symbols, I fished it and caught all three of them and some smaller ones that did not qualify for the symbols.
If is a fishing tool that is available with Humminbirds if you want to use it, I do.