Not all fish finders work well in all types of fishing. If you like to fish in freshwater then you should look for a fish finder that will work best for freshwater fishing.
In general single frequency fish finders are adequate for freshwater fishing. Single frequency fish finders work well in most lakes, rivers and in shallow coastal fishing and they are generally less expensive. Single frequency fish finders have less depth penetration than dual frequency models, but for freshwater that is generally acceptable. Most freshwater fishing is not in deep water so being able to see to depths of 1000 feet or more is not needed. The single frequency fish finders will often have a larger cone angle which will let you see a broader area at a more shallow depth.
If you happen to fish in the Great Lakes or other very deep lakes or rivers then a dual frequency fish finder will serve you better. The dual frequency settings will allow you greater visibility at deeper depth and overall deeper depth penetration.
Another aspect of freshwater fishing to consider when choosing a fish finder is what type of craft you use for fishing. Many people use kayaks, float tubes, canoes or small boats when fishing in freshwater. If you use one of these smaller crafts you should consider a portable fish finder. Both Eagle and Humminbird make portable fish finders that are both affordable and an excellent choice for shallow freshwater.
For very shallow water, two to ten feet, one of the best choices is the Humminbird Fishin Buddy line. These fishfinders are portable and also have side viewing capability which allows you to see fish off to the side of your boat. Plus you can adjust the shaft on these models to see at different depths. There are several different models in this line and they are all inexpensive. If you prefer fixed mounted fish finders, the Fishin Buddy line can also be mounted to your craft using the included bracket.
The Eagle portable fish finders are also a good option for people who fish from small crafts or from multiple boats. These portable fish finders generally have suction cup mounting systems and are quite versatile. In general the Eagle portable fish finders are also extremely affordable and definitely offer a good value for your dollar.
If you are selecting a fish finder for freshwater fishing, consider the water depth and type of craft you usually use. Unless you are fishing in truly deep water, like the Great Lakes, you can often get great results from some of the more inexpensive models. If you are in the market for your first fish finder then you may want to start with a simpler model and trade up to the more advanced models as you gain experience with finding fish easily.