I a'm confused on what type of anode to purchase. My houseboat is kept in fresh water and has twin Volvo Penta AQ200/280. I have another anode from other houseboat of mine and it seems very heavy for its size (zinc?) compared with a larger aluminum one that I have. I imagine that magnesium also is very light. Does it realy mater if A zinc or aluminum anode is used. After all the unit is kept in fresh water? Thanks.