What type of water treatment do you recommend for a closed-loop cooling system?

Every closed-loop cooling system needs a modest amount of chemical treatment. Most are used once and then checked yearly. We recommend a sodium nitrite-based inhibitor with biocide to prevent biological contamination. This treatment is usually done at a 1% concentration so not much is needed. These inhibitors will protect both the ferrous and non-ferrous materials in your piping system. Many treatment chemicals come with a colorant which allows the treatment to be visually monitored.

Source: Dry Coolers