I use a 150gpd on a constant feed to my sump which is then pumped into the main tank. There is also an overflow on my sump that goes outside and this water is stored to water the flower beds and feed a small pond. I have never used tap water on this system and its been running for 3.5 years with no problems at all.
I have to use a large amount of coral gravel in my sump to keep the Ph stable but thats about it. My water is Ph= 6.5, Gh= 3, Kh= 4,Nh4= 0, No2= 0, No3= 20ppm water is tested once a week with liquid test kits and as yet has not changed much over 3.5 years.