Your tank starts at the tap

Fill a bucket in Portland, Oregon, and you get water with almost no hardness at all, about 0.5 dGH (9 ppm). Fill the same bucket in San Antonio and it comes out around 17 dGH (300 ppm). Every water change adds your city's water to your tank, so whatever's in it is where your tank starts.

I wanted to know how different that starting point is from place to place, so I went through the public records: EPA treatment records for every community water system in the country, plus the latest water quality reports from the 500 largest utilities. Some of it surprised me.

That's a lot more people than I expected. Chlorine gases off if you let water stand for a while, but chloramine doesn't. It stays in the water until something breaks it down, like a conditioner that's rated for chloramine, and breaking it down frees a little ammonia that a cycled filter then takes care of. Plenty of chloramine utilities also switch to plain chlorine for a few weeks each year to clean out their pipes, so what comes out of your tap isn't the same all year.

Hardness, city by city

Portland, Seattle, Oakland and New York City all have soft water, under 2 dGH (36 ppm). Phoenix, Las Vegas and San Antonio sit above 12 dGH (214 ppm).

Where you live shapes what's easy to keep. Most tetras and rasboras come from soft water, and bee shrimp do best in it. Livebearers and African rift lake cichlids come from hard water. In soft-water cities, KH is the number I watch most closely: it's the water's buffering, the thing that keeps pH steady, and with very little of it pH can slide down between water changes.

What utilities add

Utilities also treat water to protect their own pipes, and some of what they add ends up in your tank. About 4 in 10 people on public water get water with phosphate added, which keeps lead and copper from leaching out of plumbing. In a tank, that same phosphate feeds algae, especially when there aren't many plants to use it up. About 1 in 5 get water that's softened at the plant, and about 1 in 6 get tap water above pH 8.5 (where the utility publishes pH), often because a higher pH cuts corrosion.

Nitrate from the tap

About 1 in 8 people get tap water with more than 10 ppm of nitrate, where the utility publishes it. That's 10 ppm the way an aquarium test kit reads it. Utility reports give nitrate as nitrogen, so the same water shows up on them as about 2.3 ppm. Water changes alone can't bring a tank's nitrate below what the new water carries, so in those places that's the floor.

Look up your city

All of this went into a free tap water lookup. Search for your city and you'll see its typical GH, KH, pH and nitrate, whether it uses chlorine or chloramine, and what those numbers mean for your fish.

• It covers US cities only, for now.
• The data has gaps. Plenty of utilities don't publish every number, and more get added as they do.
• It shows your city's typical range, not your exact tap water. Only a test of your own tap can tell you that.

The lookup assumes your tank is filled from the tap. If you log your own tap test as a tank's Source Water, or the RO, well or bottled water you actually use, I'll go by those numbers for that tank rather than your city's.