Mains water in British or city tap water in American English is highly variable across the USA!
My tap water runs non-detectable nitrates, non-detectable nitrites, pH range of 6.5 to 7, alkalinity of 5 to 20 ppm, phosphate levels of 10 to 40 ppm - a phosphate compound is added to our city tap water for control of mussels in the water system. So if I change too much water, the phosphate level is more than the pond system can "absorb", and gives me all kinds of problems, especially during Summer weather. Also, water changes does only negative things for alkalinity control, since my tap water has no significant alkalinity. So I prefer to exchange only the amount of water needed for mechanical filter cleanup/purges, and take care of the pheromones by routine low level PP treatment instead, to avoid the phosphate issue that more water changes causes in my situation. My TT's normally do okay on nitrate control with very little water exchange in the Summer with nitrates below 2 ppm, but build up to mid range (~20 ppm) in the Winter when I have such a high stocking density in the indoor koi pond at ~60F water temperature.
JR presumably has alkaline water supply free of nitrates and phosphates, so has the perfect situation for a high water exchange. And, like you say, money well spent! My own water bill goes up from ~$60 per month in Winter to ~$220 per month in the three Summer ponding months. Okay, a little of that goes to swimming pool sand filter backflushes and evaporation, but the swimming pool water exchange is small compared to the ponds - 6 outdoor ponds and two indoor ponds are running all Summer, 4 to 5 ponds for koi, 2 to 3 ponds for goldfish and golden orfes.
One of my good ponder friends has been trying to convince me that spending the money to install a well for the pond water exchange would solve nearly all the water quality problems, especially the chronic phosphate issue, and save money in the long run. After thinking through the discussion on this thread, I am beginning to believe my friend is giving unusually good advice!
Of course well or borehole water has its own special problems at times. Low dissolved oxygen, possibility of hydrogen sulfide, etc. So I would pay the money to drill the well then see if the water is actually usable from it.....
Thanks for a great discussion that got me thinking in other directions, to everyone who contributed!