The fluid's job in a water cooling system is to transport the heat from the water block to the radiator.

You might think that this article will be quite short; just add water and you'll be right. Unfortunately it isn't. There are four major considerations when selecting a fluid / fluid combination:

1. The C/W (Centigrade per Watt) of the fluid
2. The viscosity of the fluid used
3. Galvanic corrosion
4. Algae growth

The C/W of the fluid

The C/W of the fluid is important. The lower the C/W the better the fluid is at transferring heat from the water block or from the fluid to the radiator. The most commonly used fluid is water, which has a relatively high C/W in comparison to Copper. Because of this we need to physically move the warm water to the radiator to be cooled otherwise the water in the water block will become extremely hot resulting in super high CPU temps.

The viscosity of the fluid used

Viscosity; this is how runny the fluid is. The more viscous (more runny) a fluid is the better, as this will most likely result in higher flow rates.

Galvanic corrosion

Galvanic corrosion occurs when two different types of metal are present in a fluid that can conduct electricity (chemicals and metals in the water). If you have a aluminium water block and an copper radiator, the aluminum water block will break down over time and become fused to the copper radiator. Obviously this is highly undesirable. Galvanic corrosion is a real concern and shouldn't be taken lightly. I have destroyed a system due to this. Fortunately there are steps that can be taken to reduce this corrosion.

The most obvious solution to galvanic corrosion is to use only one type of metal in your system, i.e. copper radiators, copper water blocks and copper fittings. In the case of mixed metal environments, manufactures can take steps to reduce the oxidization of aluminum by anodizing it or plating it in with another more stable metal, i.e. gold. It's important to remember that anodizing/plating is only effective if the coating doesn't get scratched off leaving the bare metal exposed to the fluid.

In mixed metal systems that include anodized aluminum, it's a must to use a corrosion inhibiting additive otherwise your system will dissolve before your eyes.

Algae growth

If you use only water, at some stage it will start growing algae. This algae which is normally a white colour will of course decrease system performance by clogging water passages and coating everything, resulting in increased C/Ws. It is therefore important to use an additive to reduce this.