My Rheem hot water heater is less than a year old** and it's filling up with rust why...

I turned power off at the breaker for the hot water tank. Hooked garden hose to the drain valve located at the bottom of the tank. Shut water supply off to tank. Open relief valve on top an center of hot water heater. Then opened valve that is hooked to hose. Ran hose to street. Let the tank drain completely out. Then turned water supply back on, close relief valve an flushed tank out for maybe **mins or so until water turned to clean water. After that I closed the valve, disconnected hose an fill tank full. Turned breaker back on . Ran all house faucets until air was passed out of the lines.