Years ago, tuning your old-fashioned fishfinder was akin to tuning in TV stations, back when we used those rabbit-ears with big balls of tin-foil on the end: you’d jiggle this, turn that, then whack the side of the box and hope all the fuzz would go away. Of course, it usually didn’t. Fortunately, modern fishfinders are much better at giving you a clear picture of what lies below without any added input on your part.

fishfinder tuning

When properly tuned and zoomed in, your fishfinder will be able to tell the fish from the bottom even when they’re resting right on it.

All of those new-fangled computer chips and software patches have become very smart—smarter than most people—at figuring out which signals to ignore and which to display on your LCD. But good as they are, they are not infallible. In fact, in certain conditions your fishfinder is almost guaranteed to show you a sub-par image filled with false-positive returns, clutter, and merged targets. What can you do about it? These fishfinder tuning tips are sure to help.

1. At the start of the day, always start off in auto mode. Even in poor conditions, like water with lots of suspended solids or turbulence, auto mode will give you a starting point that’s better than your best guess.

2. If there’s a lot of clutter at the top of your screen, activate your unit’s STC (which stands for “sensitivity time control,” and basically acts as a short-range filter). That should get rid of the gobbledegook, which is usually the result of air bubbles near the surface (often created by turbulence from your boat). But, wait a sec—surface clutter can also be produced by the presence of large amounts of plankton or algae. Since these form the base of the food chain, at times you will locate fish around areas of heavy surface clutter. You might also be able to locate eddy or current barriers between different water masses by seeing a sudden and dramatic change in surface clutter -- so seeing that stuff near surface isn’t always a bad thing, and you may want to think twice before tuning it out.

3. If there’s a lot of clutter throughout the water column, you’ll probably have to adjust the unit’s gain manually. Start by using the traditional method of setting the sensitivity, tuning it all the way down until the only thing on-screen is the bottom reading. Then, advance the gain until a secondary bottom echo appears below the bottom. Now you have a good starting point. From here, if the screen is relatively clean, leave it be. If there’s clutter at the top only, hit that STC setting. And if there’s clutter throughout, you’ll have to tone down the sensitivity some more—but bear in mind that turning it down from this point may also eliminate weak returns created by small fish, weeds, and other items.

4. Don’t try to tune out faint lines that appear underwater at a relatively consistent depth. These aren’t clutter or false-positive readings, they’re thermoclines. These are temperature barriers, which have formed between different layers of water. Sometimes thermoclines show up as relatively thick lines (usually in blue or green on color units); other times, they’ll be a series of single dots. It’s important to keep track of thermocline depths because fish often gather just above or below them. (Fishing tip: Areas where drop-offs, ledges, and other structures intersect with the thermocline are often red-hot). Sensitive units will usually show thermoclines while set in auto mode, but if you have a very inexpensive or weak unit, you may have to increase sensitivity in order for them to appear.

5. Learn how the changes you make on-screen relate to the real world through  experimentation. You can do this by changing the tuning when you run into a big school of fish, and as a result you know what you’re looking at on-screen—but it could be quite a while before this opportunity arises. A more sure-fire approach is to position your boat where you know what the bottom composition is. Then alter the settings as you watch the screen, and see how it changes the bottom line. Remember that the way bottom composition is reflected on-screen seems bass-ackwards to most people. Hard bottom reflects a thin, well-defined line (which people often interpret as a weak return, but actually reflects the fact that the sonar signal didn’t penetrate deep into the bottom). Soft bottom, on the other hand, tends to leave behind a thick return which looks strong on the screen but is really the result of the sonar’s penetration into muck and mud.

6. Use your zoom function as much as possible, especially in deep water. In 200 to 300 feet of water, it’s easy to glance at the screen and think there’s nothing down there. But zoom in, and you might see swarms of fish. Why? Because your LCD screen has a limited number of pixels, so the amount of coverage each and every pixel represents increases as depth increases. When it gets to the point that a single pixel represents a foot or two of water, fish and structure, bottom, or multiple fish will all begin to merge on-screen. Just how deep will target separation begin to fail you? That depends on your specific model and the size and resolution of its screen.

-Lenny Rudow

Ready for some tips on seeing better above the surface? Check out Marine Electronics 101: How to use Radar

Written by: Lenny Rudow
With over two decades of experience in marine journalism, Lenny Rudow has contributed to publications including YachtWorld,, Boating Magazine, Marlin Magazine, Boating World, Saltwater Sportsman, Texas Fish & Game, and many others. Lenny is a graduate of the Westlawn School of Yacht Design, and he has won numerous BWI and OWAA writing awards.