Worked at a water treatment plant for nearly 30 years. One thing I see people neglect is adding hard water to a radiator or battery. Just as you know you can dissolve a tablespoon of sugar in a cup of water. You can no longer see the sugar, but you know it is in there. There is calcium in drinking water that you can not see. This calcuim will come out of solution and leave deposits every where. Use disstilled water unless you know you have soft tap water.
We have a very soft drinking water. When I flush the cooling system I will fill and drain it 5 or 6 times. Each time allowing the engine to come up to operating temp. On the last drain I will use a clear glass to catch a sample of the flush water. Just hold it up to a light. You will be suprised at what you will see.