Clean and rinse previously used water cooling equipment before using it.
Never use distilled water alone as your main coolant! Pure distilled water offers no protection against biological growth or corrosion.
Always use market proven pre-mix coolant, such as EK-Ekoolant. This propylene glycol based coolant contains all the necassery biological inhibitors, acts as a surfactant and also offers lubricating properties which enhances the lifespan of the water pump. Besides the stated it also offers corrosion protection when used in mixed-metal liquid cooling loop.
Refrain from using strips of Silver (Ag), also known as "Kill Coil" with distilled water alone in your water cooling loop. Besides introducing another metal in the already mixed-metal loop with no corrosion protection, the concentration of Silver ions may not be enough to prevent or to suppress biological growth in the liquid cooling loop.
Refrain from using Copper (II) Sulphate (CuSO4) additive - common trade name "Nuke Cu" or "Biocide Cu" - due to it's nature to react with metals usually found in liquid cooling loop, especially radiators (Zn, Cu, Sn) as well as Aluminum (Al) thus promoting corrosion. Using CuSO4 also accellerates visually discouraging copper tarnishing phenomena.
Always clean, rinse and dry water cooling equipment prior to storing it for later use!