Your Hot Tub Water Source, and What It Means
Two owners can follow the same routine and get completely different results, and the reason is usually the tap. Your fill water arrives with a chemistry of its own, and everything you do afterward is a correction from that starting point rather than from neutral.
Knowing what your source gives you means you stop fighting the same problem every fill and start correcting it once, deliberately.
Not sure where your water is? Check your readings.
What to look at, whatever your source
Four things determine how much work a source will be.
- Calcium hardness. The one most owners never check and the one that varies most. Soft water is under-saturated, meaning it has capacity to dissolve more than it holds, so it draws minerals out of whatever it touches
- Total alkalinity. Decides whether pH will hold or drift
- pH. Where the water starts, and how far you have to move it
- What else is in it. Metals, organic matter, sediment or salt, none of which show up on a standard strip
Test the source itself before you commit a tub to it. A jug from the tap tested on the counter takes a minute and tells you what you are working with.
Municipal water
Usually the least work, and worth verifying anyway.
Treated supplies are generally close to balanced for pH and alkalinity, which is why most published hot tub advice assumes them. Hardness is where they vary enormously: some regions supply water at 200 ppm and above, others as low as 10 ppm, and the advice that works in one is wrong in the other.
Your municipality publishes an annual water quality report, which gives hardness for your area without any testing. It is the fastest way to know whether you are starting hard or soft.
What to do: filter on the way in, since municipal water carries chlorine by design and that is not what you want as your sanitizer. Check hardness once and act on it, then treat the source as consistent, because it will be.
Well water
Workable and common, and the most variable source there is.
Well water reflects the ground it comes from, so two properties a mile apart can be entirely different. Hardness is often high, sometimes very high. pH runs high in limestone areas and low in others.
The part that catches people out is what else is dissolved in it. Iron is common and shows as a rust tint or brown staining once it oxidizes. Manganese does the same in black or purple. Copper matters most, because it is incompatible with a metal interior and can tint water green in a way that looks like algae but is not. Sulfur arrives smelling of rotten eggs before anything has had a chance to grow.
What to do: test every fill rather than assuming, since wells shift seasonally with rainfall and water table. Test for iron and copper as well as the standard four. Use a hose filter as standard. If hardness is very high, blend with another source rather than trying to soften it.
Rainwater
Free, soft, and the source that needs the most attention.
Rainwater arrives with almost no calcium hardness, almost no alkalinity, and a pH on the acidic side. All three of those pull the same way: the water is under-saturated and chemically aggressive until corrected, and it has no buffer to hold pH steady while you correct it.
It also carries whatever it ran across. Roof-collected water brings organic matter, and a collection surface under trees brings considerably more.
What to do: correct it on every fill rather than occasionally, since rainwater carries nothing over. Alkalinity first, because without it pH will not stay where you put it, then hardness, which is the reading protecting your tub. Filter on the way in for the organic load. Let sediment settle in the tank before drawing from it if you can.
Lake and river water
Practical at a remote property, and mostly a question of what is suspended in it.
Fresh surface water is usually moderate on hardness and alkalinity, which makes it easier to work with than rainwater. What it carries is sediment, organic matter and seasonal variation: spring melt is different from late summer, and heavy rain changes it within hours.
What to do: filter on the way in, which matters more here than with any piped source. Draw from below the surface but well above the bottom, since the top holds floating debris and the bottom holds silt. Test before soaking, always, and expect the numbers to differ between seasons.
Untreated water drawn from a lake or river can go back to it when you drain, which makes this the cleanest loop available at a remote property. More in handling water access at a remote property.
Ocean water
Only with a tub built for it, and managed by exposure rather than treatment.
Salt water needs a marine-grade structure. In a tub not specified for it, this is not a maintenance question, it is a damage question, so check before you fill rather than after.
Where it is suitable, the approach is different from every other source: you are not balancing salt water to freshwater targets, you are limiting how long it sits. Change ocean fills every five to seven days rather than holding them, and rinse the tub with fresh water between fills. Leaving salt water standing, or leaving residue on the surfaces when the tub is emptied, is what causes problems rather than the salt itself.
Drain it back to the ocean. Never onto a garden, since salt accumulates in soil and does not filter out.
More in saltwater against freshwater.
Water through a softener
Avoid it, and this is the clearest recommendation on the page.
A softener removes calcium as its entire function, replacing it with sodium. That leaves the water below range before anything else has happened, and water low in calcium draws minerals out of the surfaces it contacts.
Softened water also foams readily, because dissolved calcium is part of what raises surface tension. A tub that foams from the first fill with nothing obvious to blame is very often a softener.
What to do: find an unsoftened tap, often an outdoor spigot or one plumbed ahead of the softener, or bypass the softener for the fill. If neither is possible, correct hardness deliberately on every fill and expect to keep doing it.
At a glance
| Source | Usual character | Watch for |
|---|---|---|
| Municipal | Close to balanced, hardness varies by region | Chlorine, and unexpectedly soft supplies |
| Well | Highly variable, often hard | Iron, copper, manganese, sulfur, seasonal shift |
| Rainwater | Very soft, unbuffered, slightly acidic | Aggressive until corrected, organic load |
| Lake or river | Moderate, seasonal | Sediment and organic matter |
| Ocean | Salt, needs a suitable tub | Change every 5 to 7 days, rinse between fills |
| Softened | Stripped of calcium | Aggressive and prone to foaming. Bypass it |
Mixing sources
Worth knowing, because it explains readings that make no sense. Topping up a municipal fill with well water gives you a chemistry that matches neither, and a tub topped up repeatedly from a different source drifts somewhere unpredictable.
Blending deliberately is a legitimate technique, particularly for diluting very hard well water. Blending accidentally is how people end up chasing a number all season. Retest after any significant top-up from a different source.
If you have an AlumiTub
The marine-grade aluminum structure is what makes the range of sources possible, including ocean fills that most tubs cannot take at all. What it asks in return is that calcium hardness stays in range, because water below roughly 100 ppm is under-saturated and chemically aggressive toward the interior.
That makes two sources worth particular care. Rainwater and softened water both arrive with almost no calcium and both need correcting on every fill rather than occasionally. If either is your only supply, budget for that as part of the routine rather than treating it as an exception.
On well water, test for copper as well as the standard four, since copper is incompatible with the interior and is common in some supplies.
The published targets are alkalinity 80 to 120 ppm corrected first, pH 7.4 to 7.8, calcium hardness 150 to 250 ppm, and bromine 2 to 3 ppm and never above 3. The Purifier hose filter in Good Clean Living Water Care is built for the fill step and is worth using whatever your source. More on the mechanism in calcium hardness and scale, and on filling in filling a hot tub well.
Not sure where your water is? Check your readings.