How do i fill my tank up with water if my city tap water is too hard for fish??

short answer Reverse Osmosis prefiltered water reconstituted to the proper Ph. But before you go to extreem measures, how hard is your water and what fish do you intend to keep?

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Gold Coast Aquarium Society South Florida
www.goldcoastaquarium.org

well i haven't done an official test but i've heard from other people that water where i live is extremely hard...i just want to raise like guppies or something like that, last time i tried, fish would all eventually die no matter how perfect every other aspect was.

you could look into african cichlids they love extremely hard water

Where I live, we too have very hard water. So I used the water from my plastic rain barrel. My fish tank is in the basement, so I ran a hose from the barrel to the tank, and let gravity do all the work.
My pH levels are around 120 (GH)ppm, and my Alkalinity is near 300 (KH)ppm. With winter fast approaching here in Ontario, and the rain barrel drained until spring, I've been experimenting using the water from my dehumidifyer to fill up my tank after water changes. So far everything is going well. The calcium buildup that was always left on the glass from tap water evaporating is non-existent now. The water still evaporates, but the glass is crystal clear when the water goes. I don't think my tank will ever see one drop of tap water again. The one concern I had with using the rain barrel was any chemicals from the shingles that may leach into the water supply. As far as I can tell, if any are, they are not affecting the fish or plants in any harmfull way. I have neon tetra's in there, and I have been told they can be hard to keep sometimes. They all seem to be doing well. I hope this helps give you an option for your water problem.
P.S. I bought my rain barrel at Home Depot for $70.