Best answer: It depends on your PC build and the design you're aiming for. All you truly need for a water cooling loop is distilled water and a biocide. This is the core foundation of your coolant, which can handle your chosen dye should you desire a colored finish.
Water cooling biocide: PrimoChill Liquid Utopia ($14)
Distilled water is your water cooling friend
At the very least, all you need is distilled water. That's more than enough for your pump to move the liquid around and take heat from the blocks to the radiator. Why the biocide is added is to help protect all your parts from corrosion and prevent bacterial growth. It also won't color the water, which results in a clear liquid.
With distilled water and biocide, you now have a complete loop and should be good for 6–12 months when you'll need to flush out and refill the system. It's best to buy distilled water from a nearby store as pricing online can be vastly higher. Should you wish to add some color to your water cooling loop, you'll need to choose a dye.
Premixed solutions for unique finishes
When clear or slightly tinted liquid simply won't do in your PC build, you may want to consider some premixed solutions from companies like PromoChill and Mayhems. Numerous vendors offer premixed formula with various visual results. For instance, Mayhem's Pastel family of liquids are popular among PC builders for vibrant solid colors.
These solutions are ready-to-use and do not require any additional biocide — simply ensure the water cooling loop is clean and pour into the reservoir. There are some minor issues with some liquids, include the blockage of block fins and discoloration of tubing. Adequate research is required before purchasing a premixed formula to make sure all your fittings and parts are compatible.
Little choice with AIO coolers
Opting to go with a pre-configured and sealed all-in-one (AIO) water cooling solution means you don't have to deal with the initial setup process. Using such a cooler does come with a few drawbacks, however. The first is having no choice on the coolant used in the loop. It's down to what the vendor chooses.
Secondly, you have no way of draining the loop to replace or top-up the liquid, though you shouldn't have to do this if you plan on switching out within a few years. The majority of AIO coolers use black plastic tubing, which means you won't see liquid moving around the loop and thus it's usually not dyed.
Keep the loop clean
Biocide is an important component for any water cooling loop. Not only does it prevent growth from taking over your cooling solution, but also helps protect against metal corrosion.
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