If you have metered water then the price of the water will reflect it's environmental value so you don't need to worry about wasting resources if you are billed. You only need to worry about the bill.
It would be interesting to compare the water bill for the cooling to the electricity cooling bill and water cooling investment. $100 for a rad and pump should buy you a lot of water.
If you already have a steady consumption of water for one reason or other, then the water will be free since you aren't degrading it by running it through your waterblock first.
You don't need a huge flow. 1 L/min is enough. Most showers use more than 10 L/min so your shower won't be effected.
Also with tap water your computer won't heat your room since the heat goes down the drain as opposed to heating the room. If you had a lot of computers in a room it might be a realy good idea to just cool with tap water so that the room doesn't get too hot.
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