The faster they erode the better!!
also the proper ones will be zinc, i wouldn't get aluminium ones as they are supposed to be sacrificial meaning they will rust instead of your engine!, Zinc is more reactive then aluminium so is more likely to save your engine.
also the aftermarket aluminium anode would not really be much good because the leg/gearbox is made of aluminium so the anode wouldn't be sacrificial, no more then your leg/gearbox is!
thats where i get all the stuff for our engine. fast and cheap