Hello,The "rust" is from the water itself.If the supply has a high iron content the process of heating it up seperates the iron out and over time it will form deposits on the bottom of the hot water tank.Locate the tank drain and connect a hose and open it up and let it run "flush" out the tank.Do this every month to keep it from building back up.It should be mentioned that the drain valve can be plastic and or very hard to close once it has been opened.They sell a brass drain end cap or the drain can be replaced with a new (long stem)drain.