Cupertino, CA tap water
Supplied mostly by Santa Clara Valley Water District, plus 3 other utilities. Here's what the water means for your fish.
Disinfectant
Chloramine
Your water is treated with chloramine. Unlike chlorine, it doesn't gas off when water sits out, and many pitcher and fridge filters aren't rated to remove it. Conditioners that list chloramine on the label break it apart, which frees about 0.34 ppm of ammonia here. Some utilities switch to plain chlorine for a few weeks a year.
Safety for fish and shrimp
Ammonia (with conditioner)
Estimate0.34 ppm (Low)
This is what a conditioner frees when it breaks the chloramine apart. A cycled filter clears it within a day.
Estimated from the chloramine level San Jose Water reports (2025).
Nitrite (NO₂)
Not detected (Low)
Not detected in testing, so any nitrite in your tank comes from the tank itself.
From California Water Board lab results; EPA monitoring data (2016–2026).
Nitrate (NO₃)
1.5–32 ppm (Low)
Typical 1.5 ppm
Low, so water changes lower your tank's nitrate well.
From California Water Board lab results; San Jose Water's 2025 water quality report; EPA monitoring data; Santa Clara Valley Water District, which supplies City of Cupertino (2013–2026).
Copper at home taps
Up to 1.4 ppm (Low)
Typically not detected
Not detected in testing. Water that sits in household pipes overnight can still pick some up.
From California Water Board lab results; San Jose Water's 2025 water quality report (2025–2026).
Chlorine or chloramine at the tap
1.4 ppm
At tap levels, chlorine and chloramine are toxic to fish, shrimp and the bacteria in a filter. Water conditioners are made for this range.
Published by your utility: San Jose Water's 2025 water quality report.
Water chemistry
General hardness
99–452 ppm
Typical 102 ppm
Moderately soft to medium. Suits most community fish and plants.
From California Water Board lab results; San Jose Water's 2025 water quality report; Santa Clara Valley Water District, which supplies Cwsc Los Altos Suburban; Santa Clara Valley Water District, which supplies City of Cupertino (2025–2026).
Carbonate hardness (alkalinity)
68–140 ppm
Typical 68 ppm
Moderate buffering. pH should stay fairly steady.
From California Water Board lab results; Santa Clara Valley Water District, which supplies Cwsc Los Altos Suburban; Santa Clara Valley Water District, which supplies City of Cupertino (2023–2026).
pH
7.4–8.2
Typical 7.6
Most community fish do well here. Stable water matters more than the exact number.
From California Water Board lab results; EPA monitoring data; Santa Clara Valley Water District, which supplies City of Cupertino (2012–2026).
Total dissolved solids
180–520 ppm
Typical 279 ppm
A rough measure of everything dissolved in the water. RO water reads near 0, so TDS is a quick way to compare waters.
From California Water Board lab results; San Jose Water's 2025 water quality report; Santa Clara Valley Water District, which supplies Cwsc Los Altos Suburban; Santa Clara Valley Water District, which supplies City of Cupertino (2025–2026).
- Santa Clara Valley Water District, San Jose Water, Cwsc Los Altos Suburban and City of Cupertino add phosphate to protect pipes. It can feed algae in tanks without much plant growth.
Who supplies Cupertino, CA
Santa Clara Valley Water District
Serves 51% of residents · Lake or river water · Chloramine
- 2025 water quality report
San Jose Water
Serves 21% of residents · Lake or river water · Chloramine
Cwsc Los Altos Suburban
Serves 17% of residents · Buys water from Santa Clara Valley Water District · Chloramine
City of Cupertino
Serves 14% of residents · Buys water from Santa Clara Valley Water District · Chloramine
Our data comes from public records and is still incomplete. Many utilities don't publish every number, so we're adding more all the time. Your city's range is a starting point: testing your own tap is the only way to know exactly what's in it.
Tap water lookup covers US cities only for now. Want to know why it matters? Read our story on tap water.
Compiled by Shoal & Stem from public records. Free to cite with a link.