Protect Water Tank From Corrosion

What is hard water?
Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum which are largely made up of calcium and magnesium carbonates, bicarbonates and sulfates.
Hard drinking water may have moderate health benefits, but can pose critical problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling towers, and other equipment that handles water. In domestic settings, hard water is often indicated by a lack of foam formation when soap is agitated in water, and by the formation of limescale in kettles and water heaters. Wherever water hardness is a concern, water softening is commonly used to reduce hard water's adverse effects.
Hard water areas generally have > 280ppm.

What is soft water?
Soft water is surface water that contains low concentrations of ions and in particular is low in ions of calcium and magnesium. Soft water naturally occurs where rainfall and the drainage basin of rivers are formed of hard, impervious and calcium-poor rocks.
Because soft water has few calcium ions, there is no inhibition of the lathering action of soaps and no soap scum is formed in normal washing. Similarly, soft water produces no calcium deposits in water heating systems. Water that isn't soft is referred to as hard water.
Soft water areas generally have < 100ppm.

But why does this matter?
The conductivity of the water effects the method of protection you choose for your water heater.  
Choosing anodes to protect your hot water tank from corrosion - comparing sacrificial anodes and powered anodes
What is a sacrificial anode?
A sacrificial anode by virtue of its name ‘sacrifices’ itself for the benefit of the hot water tank. Using the principles of electrochemical processes when two different metals are immersed in water one will corrode away to protect the other. Sacrificial magnesium anodes are used in hot water tank acting as the less noble metal and corrodes to protect the inner tank of the hot water tank.
Sometimes the quality of water prevents magnesium anodes from corroding. These water conditions are typically soft water. The low electrical conductivity in soft water reduces electrical flow from anode to cathode (inner tank) through the water. This stops the anode from corroding and therefore won’t protect the inner tank.
When inspecting the anode in these applications it could appear in excellent condition. However this may be an indication that the anode is not working and the heater is at risk.
What is a powered anode?
Powered anodes, or impressed current anodes, are non-sacrificial. The metal of the anode is titanium and theoretically will last the lifetime of the hot water tank.

Powered anodes use an electrical supply to produce a very low current into the water to replace the electrolytic current produced by sacrificial anodes.

It is good practice to remove the sacrificial anode (if already installed) from the hot water tank to maximise protection.

Which anode protection should I use for my hot water tank?
It depends on the area of the you are in and your choice of hot water tank.
Things to consider when choosing anode protection for your water tank:
Hardness of the water
Sacrificial anodes often don’t work in soft water areas due to the low electrical conductivity. But a powered anode in certain soft water areas may also have no effect in protecting the cylinder shell as the conductivity of the water, which the powered anode requires, may still be too low.

Site conditions
If the site is prone to electrical supply failures then powered anodes should not be used as without a power supply the hot water cylinder will not be protected from corrosion.

Cost and installation
Electrical anode protection can be more expensive initially due to a higher purchase price and the additional electrical connections that are needed for installation. However, over the lifetime of the unit they could be more cost effective than maintaining and replacing sacrificial anodes.

Again, all this needs to be considered in relation to the site and individual project.

Electrical supply and connections are needed for powered anodes. However powered anodes come with an indicator red or green LED to help you identify the status of the protection system.

Sacrificial anodes require maintenance and will corrode at different rates depending on the location. Regular inspection is required by an engineer to check the state of the anodes. You will need to learn how quickly the anodes deteriorate and therefore how often they need changing.

Remember, the sole purpose of an anode is to protect against corrosion and does not have any effect on reducing scale or legionella in the hot water tank.