ethanol fuel doesn't have that brown stuff in it. But if you've got an older boat that used to run old style gas without ethanol you'll get a varnish build up in the tank like a scum line where the fuel is filled to in the tank. Not a problem as long as you always use ethanol free fuel, problem arises when you put fuel with ethanol in the tank it ends up loosening up that varnish. Or so I've heard. You should have another filter under the cowling that should also be replaced.
