San Diego's tap water is hard, alkaline, and treated with chloramine rather than plain chlorine — and all three of those facts change how you should set up and maintain a freshwater tank here. This isn't a guess: it's pulled from the City of San Diego Public Utilities Department's own 2025 annual drinking water quality report. Below is what the numbers actually are, and what they mean for stocking, dechlorinating, and whether you need an RO/DI setup at all.
San Diego's tap water, at a glance
| Characteristic | San Diego's reported range | What that means for a tank |
|---|---|---|
| Total hardness | ~200-285 ppm as CaCO₃ (roughly 11-16 dGH) | Hard to very hard — above the 4-12 dGH range most community-tank charts call typical |
| pH | 7.56-8.64, plant averages 8.07-8.40 | Alkaline — comfortable for hard-water fish, firmly outside soft-water-species territory |
| TDS (total dissolved solids) | ~430-630 ppm, averages roughly 530-585 ppm | On the high side; a useful baseline number for a TDS meter |
| Disinfectant | Chloramine (secondary, distribution-wide) | Needs a dechlorinator rated for chloramine, not just chlorine |
Figures are from the City of San Diego Public Utilities Department's 2025 Annual Drinking Water Quality Report and its accompanying water-characteristics data, covering the Alvarado, Miramar, and Otay treatment plants that supply most of the city. Your exact numbers depend on which plant serves your neighborhood and the time of year — see water parameters and testing for how to check your own tap directly rather than relying on a citywide average.
Why it's this hard: where the water comes from
Most of San Diego's water is imported through the San Diego County Water Authority as a blend of Colorado River water and State Water Project supplies, both naturally mineral-rich, with less than 10% coming from local sources like the Carlsbad Desalination Plant and Twin Oaks Valley treatment plant. Colorado River water in particular is well known for its hardness — it's the same regional supply behind the hard water complaints common across much of Southern California and the Southwest, not something specific to San Diego's own treatment process.
What hard, alkaline water means for stocking
San Diego's water profile fits some popular freshwater fish very well and fights against others. It's worth knowing which camp your livestock is in before you stock, not after.
- Naturally well-suited: livebearers (guppies, mollies, platies, swordtails), most goldfish (see our goldfish vs. tropical fish guide for why goldfish need their own setup anyway), and the great majority of hardy community fish sold locally.
- Fine without much thought: most beginner community species covered in our beginner freshwater fish guide — tolerant of a fairly wide hardness and pH band.
- Needs adjustment: soft-water specialists — wild-caught blackwater species, some dwarf cichlids, and Caridina shrimp — which want meaningfully softer, more acidic water than anything coming out of a San Diego tap.
Shrimp: Neocaridina is lucky, Caridina isn't
This is the clearest local example of why these numbers matter. Our Neocaridina shrimp care guide puts their ideal GH at roughly 6-12 dGH — meaning San Diego's naturally hard tap water, once dechlorinated, sits close to perfect for cherry shrimp and their color variants without any remineralizing at all. It's one of the quieter reasons Neocaridina do so well for local keepers straight from the tap.
Caridina shrimp (crystal red, black shrimp, Taiwan bees) are the opposite case. They need soft, more acidic water well below San Diego's natural hardness and pH, which means San Diego tap water is actively the wrong water for them as-is. Keeping Caridina here typically means running the tap through an RO/DI unit and rebuilding it with a Caridina-specific remineralizer — not just diluting tap water with RO, which doesn't reliably hit the narrow band they need.
Don't use a home water softener's output for your aquarium. Household softeners work by ion exchange — swapping calcium and magnesium for sodium — which doesn't lower hardness the way fish need and pushes sodium to a level that can stress or kill livestock over time. If your plumbing runs through a softener loop, draw water-change water from an outdoor spigot or another tap that bypasses it.
Chloramine: the detail a lot of dechlorinators miss
San Diego's treatment plants use ozone or chlorine dioxide for primary disinfection, but chloramine is added afterward for secondary disinfection — the residual that keeps the water safe all the way through the distribution system to your tap. That means the chlorine/chloramine section of our water parameters guide applies directly here: a dechlorinator that only neutralizes chlorine will leave the chloramine's ammonia component behind. Check the label for an explicit chloramine claim, not just "removes chlorine."
pH and KH: stable, just on the alkaline side
San Diego's reported pH runs from about 7.56 to 8.64, with plant averages in the 8.0-8.4 range — noticeably more alkaline than the 6.5-7.5 most community-tank charts list as typical. The upside is stability: hardness this high comes with plenty of carbonate buffering (KH), so San Diego tap water resists the sudden pH crashes that plague very soft, low-KH setups elsewhere. The tradeoff is that it's firmly unsuitable, untreated, for anything that specifically wants a low, soft-water pH.
Do you need an RO/DI system?
For most San Diego fishkeepers, no. If your stocking list is community fish, livebearers, goldfish, or Neocaridina shrimp, dechlorinated tap water is appropriate water, not a compromise. RO/DI earns its cost specifically for soft-water specialists — Caridina shrimp, some wild-caught South American and Southeast Asian fish, and a handful of demanding plant species — where San Diego's natural hardness and pH work against the animal rather than for it.
Is San Diego tap water hard or soft?
Hard to very hard. The City of San Diego's own 2025 water quality report puts total hardness at roughly 200-285 ppm as calcium carbonate across its three treatment plants, which works out to about 11-16 dGH — well above the 4-12 dGH most community-tank guides call typical.
Does San Diego use chlorine or chloramine?
Chloramine. The city's treatment plants use ozone or chlorine dioxide for primary disinfection, but chloramine is added for secondary disinfection to keep a residual through the distribution system — so the water reaching your tap carries chloramine, not plain chlorine. Use a dechlorinator labeled to handle chloramine, not just chlorine.
Is San Diego's tap water good for shrimp?
It depends which shrimp. Neocaridina (cherry, blue dream, yellow fire) want roughly 6-12 dGH, so San Diego tap water — once dechlorinated — sits close to their range without remineralizing. Caridina (crystal red, Taiwan bee) need soft, more acidic water well below San Diego's natural hardness and pH, so they typically need RO/DI water built back up with a Caridina-specific remineralizer.
Can I use water from my home's water softener for my aquarium?
No. A household water softener works by ion exchange, swapping calcium and magnesium for sodium — it doesn't lower hardness the way an aquarium needs, and it raises sodium to a level that can stress or kill fish and shrimp over time. Draw aquarium water from an outdoor spigot or another tap that bypasses the softener loop.
Do I need an RO/DI system for a community fish tank in San Diego?
Usually not. Most community fish, livebearers, and Neocaridina shrimp tolerate or even prefer San Diego's naturally hard, alkaline water once it's dechlorinated. RO/DI is worth the cost mainly for soft-water specialists — Caridina shrimp, wild-caught blackwater fish, and some delicate plants — not for a typical beginner community tank.
Does San Diego's water hardness or pH change over time?
A bit, seasonally and by treatment plant — the city's own report shows hardness and pH ranges, not single fixed numbers. It's worth testing your own tap once to get a baseline, and retesting if your water utility announces a treatment change or if you move to a different part of the county.