That is indeed a very difficult task. I've been having the same problem at my local river with hard bottom and rocky banks. It might be impossible, actually. The best condition to see fish is a structureless soft bottom. They will then show up as stripes with shadows. But as soon as the fish sit too close to the bottom, it becomes virtually impossible. The sonar sees "hard" objects, but doesn't know why they are hard. Air? Stone? Metal? Impossible to tell. The only way to tell is than usually by a) checking if they are stationary and b) if they fit to the environment: e.g. if they are hard echos in a part of the lake where there are boulders scattered, they are more likely stones. If they are in a lat area and close together, they might rather be a swarm of fish.
Maybe you can upload some images and people can give it a shot.