I'm real happy with my Matrix 12 but if I were to replace it right now it would be with a Humminbird 535. It's also uses a single beam, 20-degree transducer but has more pixels and should have a little better resolution.
Once you get your sensitivity set right you shouldn't have any trouble seeing fish.
But if you're sitting still or moving very slowly don't expect to see defined arches - they will show up as long lines that may move up and down the screen (if the fish is moving up and down in the water column) or they may be just a solid line (if the fish is staying at the same level and staying in the view of the trandsducer).
Once you can identify the sonar returns try using the Fish ID with just enough sensitivity to show a fish symbol and target depth on the obvious "fish" returns if it will. If you can't get it to show both the actual sonar returns and fish symbols then you probably want to run it with Fish ID off.