Diamond Bar, CA tap water
Supplied mostly by Three Valleys MWD, plus 1 other utility. 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 0.51 ppm of ammonia here. Some utilities switch to plain chlorine for a few weeks a year.
Safety for fish and shrimp
Ammonia (with conditioner)
Estimate0.44–0.58 ppm (Worth knowing)
Typical 0.51 ppm
This is what a conditioner frees when it breaks the chloramine apart. A cycled filter clears it within a day; a big water change in a new or very small tank adds more of it at once. Some conditioners also bind ammonia for a day or so.
Estimated from the chloramine level Walnut Valley Water District 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 (2023–2026).
Nitrate (NO₃)
Up to 16 ppm (Low)
Typical 0.3 ppm
Low, so water changes lower your tank's nitrate well.
From California Water Board lab results; Walnut Valley Water District's 2025 water quality report (2023–2026).
Copper at home taps
Up to 0.74 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; Walnut Valley Water District's 2025 water quality report (2023–2026).
Chlorine or chloramine at the tap
1.8–2.4 ppm
Typical 2.1 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: Walnut Valley Water District's 2025 water quality report.
Water chemistry
General hardness
97–280 ppm
Typical 99 ppm
Moderately soft to medium. Suits most community fish and plants.
From California Water Board lab results; Walnut Valley Water District's 2025 water quality report (2023–2026).
Carbonate hardness (alkalinity)
72–220 ppm
Typical 72 ppm
Moderate buffering. pH should stay fairly steady.
From California Water Board lab results; Walnut Valley Water District's 2025 water quality report (2023–2026).
pH
7.7–8.6
Typical 8.2
Alkaline. Many utilities raise pH to protect pipes. Stability matters more than the exact number.
From California Water Board lab results; Walnut Valley Water District's 2025 water quality report (2023–2026).
Total dissolved solids
250–660 ppm
Typical 250 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; Walnut Valley Water District's 2025 water quality report (2023–2026).
Who supplies Diamond Bar, CA
Three Valleys MWD
Serves 52% of residents · Lake or river water · Chloramine
- 2025 water quality report
Walnut Valley Water District
Serves 52% 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.
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.