Save Your Solar Water Heater Tank. Sacrifice Your Anode!
You probably don't replace the magnesium rod which came in your current solar hot water heater tank. Most people don't.
It's called a "sacrificial anode" and is probably the most important factor in whether a water heater lives or dies. For more than half a century, it's has been used as a key part of the rust protection of a tank. Most people don't even know it's there.
The rod is typically made of magnesium and screwed into the top of the tank.
When the tank is filled with water, an electrolytic process consumes the magnesium rod (anode) to protect a small amount of exposed steel.
All metals are galvanically reactive, some more than others. When two are placed together in water, the "nobler" -- or less reactive -- one will remain intact while the more reactive one corrodes. Magnesium is less noble than steel, which is why it's used for the anode rod.
The solar water heater tank comes with a magnesium rod installed. However, it will wear away over time.
If your water quality is poor, e.g. well water, or highly salty, you should replace it yearly. Treated municipal water does not have as many minerals or other contaminants, so you could replace it every 2 years.
Replacing it is simple. Turn off the water pressure and drain the solar water tank. Caution, the water in the tank will likely be very hot and can cause physical injury if allowed to contact bare skin.
Cover the collector tubes with a sheet to keep them from heating up too much.
Once the tank is empty, unscrew the old magnesium rod and inspect it. If it is mostly gone, replace it. If it is only corroded a little bit, you can screw it
in again. By noting how much has corroded, you can make a good guess as to how long it will take to completely corrode away.
Once the new rod is installed, turn the water pressure on again and check for leaks. If there are no leaks, take the sheet off the collectors, returning it to service.
New magnesium rods are not expensive, and will greatly extend the useful life of your solar hot water tank.