The permissible level of minerals and other species in cooling water depends on the specifics of the system. According to the Nalco Water Handbook, there are three water side problems encountered in cooling systems: corrosion, scale, and fouling. Corrosion is a function of water characteristics and the metals in the system. Corrosion can cause premature metal failures and deposit of corrosion products that reduce both heat transfer and flow rates. Scale is caused by precipitation of compounds that become insoluble at higher temperatures such as calcium carbonate. Just like corrosion products, scale can interfere with heat transfer and reduce flow rates. Fouling results from the settling out of suspended solids, build up of corrosion products, and growth of microbial masses. Fouling has the same effect on the system as scaling but it also promotes severe corrosion under deposits. A thorough understanding of system design, operating characteristics and water chemistry and how these interact to influence corrosion, scaling, and fouling is required before a water treatment option can be selected. As a starting point, I suggest you obtain one of the many excellent references covering this subject such as the Nalco Water Handbook.