San Mateo, CA tap water
Supplied by California Water Service - San Mateo. Here's what its 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 a little ammonia. Some utilities switch to plain chlorine for a few weeks a year.
Safety for fish and shrimp
Nitrate (NO₃)
Supplier's figureUp to 1.9 ppm (Low)
Typically not detected
Not detected in testing, so water changes lower your tank's nitrate well.
From the utility that supplies yours: San Francisco Regional Water System, which supplies California Water Service - San Mateo (2023–2025).
Copper at home taps
0.02 ppm (Low)
Low. Water that sits in household pipes overnight can carry more than this.
From California Water Board lab results (2025).
Water chemistry
General hardness
Supplier's figure8–106 ppm
Typical 70 ppm
Soft. Good for tetras, rasboras and most shrimp. Livebearers, snails and African cichlids prefer harder water.
From the utility that supplies yours: San Francisco Regional Water System, which supplies California Water Service - San Mateo (2023–2025).
Carbonate hardness (alkalinity)
Supplier's figure8–102 ppm
Typical 84 ppm
Moderate buffering. pH should stay fairly steady.
From the utility that supplies yours: San Francisco Regional Water System, which supplies California Water Service - San Mateo (2023–2025).
pH
8.6–8.7
Typical 8.7
Alkaline. Many utilities raise pH to protect pipes. Stability matters more than the exact number.
From California Water Board lab results (2026).
Total dissolved solids
Supplier's figure24–269 ppm
Typical 146 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 the utility that supplies yours: San Francisco Regional Water System, which supplies California Water Service - San Mateo (2023–2025).
Who supplies San Mateo, CA
California Water Service - San Mateo
Serves 98% of residents · Buys water from San Francisco Regional Water System · 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.
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Compiled by Shoal & Stem from public records. Free to cite with a link.