Yeah, double check your manual to be sure it is completely in manual mode. On my Raymarine unit there are about 5 steps to do that. I can put one function in manual and still have several functions working on auto. PITA in my opinion. Not sure how yours works but it may be similar.
Regarding the marks at 200 feet, I'm not sure what that could be. Does it matter what depth you are in? It may be a thermocline but they usually vary from day to day.
Another trick for setting your gain is to set the depth for a little over two times the depth you are actually in and turn up the gain till you get a slight second bottom echo down below the real bottom. Then you set it back to the proper depth and you are good to go. I had an old video unit that worked great when I took the time to set it up that way.
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You'll have to excuse me but I'm running a carbon deficit today.
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