Originally Posted by Comodave
It would be a waste. In fresh water the magnesium is the way to go. The zinc anode won’t do anything and the magnesium will do all the work. You can use aluminum in fresh water. I use aluminum in fresh water since I can’t find all of the sizes I need in magnesium.
When I moved from salt water into fresh water There was full set of new zinc anodes on all underwater portions, as well as a bonded "divers dream" plate on transom.
Every 3 to 4 months [or sooner] I go under boat and scrub all zinc surfaces clean with rigid bronze brush and stainless scraper. As the zincs slowly decreased in size [doing their surface shedding protection due to consistently cleaned surfaces] I began adding mag anodes. By next summer all anodes will be magnesium.
I've had zero trouble with boat's metal parts.
BTW - Boat is kept in an electric current cool marina and never left for any length of time [more than a few days in a row] with dock power plugged in. Also, all batteries are switched into isolation when leaving boat.