Fluids degrade in time so as a rule of the thumb, replace fluids from what is recommended on the manual. Most coolant in the market are Sulfur, Phosphate, Borate, Amine-based corrosion inhibitor, they tend to buildup deposits an cause abrassive particles that could kill the water pump. Some coolant like Dexcool uses 2-ethylhexanoic acid (2-EHA) that when improperly used, could harm plastic and rubber seals and severely corrode the engine internals.

Remember that too much coolant could buildup deposits while too little could start a corrosion to disimilar metals-aluminum, copper, and iron parts of the engine, so most manufactureres do stipulate the ratio. Most jap cars recommends 50:50 mix of distilled water and coolant concentrate replaced every 40k kms, while for some fleet users long-life coolant is used plus a recharger fluid every 50k kms.