Lake Mathews, CA tap water
Supplied by Western MWD. 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)
1.2 ppm (Low)
Low, so water changes lower your tank's nitrate well.
From California Water Board lab results (2025–2026).
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 figure8–570 ppm
Typical 197 ppm
Moderately hard. Great for livebearers, and most community fish adapt.
From the utility that supplies yours: Metropolitan Water Dist. of So. Cal. and City of Riverside and Eastern Municipal WD and Western MWD and Chino Basin Desalter Auth. - Desalter 2, which supplies Western MWD (2023–2026).
Carbonate hardness (alkalinity)
Supplier's figure9–360 ppm
Typical 133 ppm
Strong buffering. pH stays steady and hard to lower without RO water.
From the utility that supplies yours: Metropolitan Water Dist. of So. Cal. and City of Riverside and Eastern Municipal WD and Western MWD and Chino Basin Desalter Auth. - Desalter 2, which supplies Western MWD (2023–2026).
pH
8.5
Alkaline. Many utilities raise pH to protect pipes. Stability matters more than the exact number.
From California Water Board lab results (2025–2026).
Total dissolved solids
Supplier's figure27–1150 ppm
Typical 357 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: Metropolitan Water Dist. of So. Cal. and City of Riverside and Eastern Municipal WD and Western MWD and Chino Basin Desalter Auth. - Desalter 2, which supplies Western MWD (2023–2026).
- Western MWD adds phosphate to protect pipes. It can feed algae in tanks without much plant growth.
Who supplies Lake Mathews, CA
Western MWD
Serves 91% of residents · Buys water from Metropolitan Water Dist. of So. Cal. · 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.