Anode rods protect the steel wall of the tank from corrosion but they don't affect the amount of scale that forms in the tank. It's a good idea to replace your anode rod periodically, but like Glennjanie said, few ever do.
Lime scale forms on the hottest surfaces in a hot water tank. If it's an electric hot water heater, the lime scale will coat the heating elements and cause them to burn out prematurely. In a gas fired hot water heater, the lime scale will form on the bottom surface of the tank, and that causes the metal on the bottom of the tank to get hotter than it otherwise would, and that reduces the lifespan of the tank before it starts to leak.
What you're looking for is an acid flush for your hot water tank, and A. O Smith makes such a product for dissolving the lime scale in their tanks (but that can be used on any gas fired water heater.
A. O. Smith Water Heaters
hover over the "Literature" link and click on "Service Handbooks"
Click on the "Click here" link in the "Click here to download or print a .pdf"
Download the .pdf file entitled "Why? When & How? To Remove Water Scale from Tank Type Water Heaters".
Hope this helps.
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