You might want to consider investing in an underwater camera like an Aqua-vu if you're not able to read minnow concentrations on your depth-finder. It'll help you eliminate spots where the fish aren't on any given day so you don't waste a lot of time on unproductive spots. It'll also help you pinpoint rock and weed combos, usually good fishing terrain.
I use mine to quickly find the optimum depths for bass and pike fishing. When I start seeing schools of panfish swimming by, I know that's where I should be fishing.
You can also video fish with 'em, which is a lot of fun, particularly drifting with minnows on spinners. You can watch the bottom go by as you keep your bait a couple inches above it, waiting for the strike. And you cant tape 'em for future viewing if you hook your underwater camera up to a camcorder. I have a video on youtube called PIKESTRIKE! that shows a couple of drifting hits. One of my other videos - Thief Steals Sucker - has been watched over 38,000 times in a little over a year, not too shabby for an amateur video.