I put 300 hours on my engines each season. I have some routine engine maintenance to do every 300 hours.
As a matter of annual maintenance routine, I change out the internal/external zincs each season. Also change out the Impellers. This season, I have now added changing out the thermostats as they are really only good for 300 hours, same hour limit too for impellers and zincs.
Sharing some pics to show how to change out the internal zincs, and evidence as to why at 300 hours its absolutely necessary. These pics show what the internal zincs look like on the engine block (second pic from left), and its a simple job to remove the bolt holding the zinc unit in and slowly twist to remove it. Be mindful of the "o" ring seal - to not damage it. Then remove the old zinc and replace with new one (far right pic).
Included some pics showing how badly the internal zinc's wear away (doing their job). Pretty clear why they should be changed out at 300 hours.