How do you fill a hot tub properly?

Start with a clean tub and a clean filter, flush the hose, and fill through a hose filter to remove chlorine, metals and sediment before they reach the water. Then balance in order: total alkalinity first, then pH, then calcium hardness, then sanitizer last.

Should you add chemicals straight after filling a hot tub?

Not sanitizer, and not all at once. Correct total alkalinity first because it buffers pH, then pH, then calcium hardness, and only then sanitizer. Sanitizer added to unbalanced water is far less effective, so adding it first means using more to less effect.

Can you fill a hot tub with rainwater?

Yes, but correct it on every fill rather than occasionally. Rainwater arrives with almost no calcium hardness, almost no alkalinity and a slightly acidic pH, which makes it under-saturated and chemically aggressive until corrected. Filter it on the way in, since roof-collected water carries organic matter.

Can you fill a hot tub through a water softener?

Avoid it. A softener removes calcium as its entire function, so the water arrives below range before anything else has happened, and water low in calcium draws minerals out of the surfaces it touches. Use an unsoftened tap or bypass the softener for the fill.

How soon should you test water after filling a hot tub?

Test as you balance, then two or three times during the first week rather than waiting for your normal schedule. New water moves more than settled water, so a fill that looked right on day one can drift by day three as everything reaches equilibrium.

How to Fill a Hot Tub

By AlumiTubsAugust 26, 2026

A fill takes twenty minutes and decides how the next three months go. Water that starts in range stays in range longer, needs less sanitizer, and gives you far less to correct later. Water that starts wrong is something you spend weeks chasing.

The good news is that filling well is mostly a matter of order and one piece of equipment.

Not sure where your water is? Check your readings.

Before the hose goes in

Know your source. This is the step almost everyone skips, and it is the one that explains most first-fill problems. Municipal water varies enormously by region. Well water varies by property. Rainwater arrives with almost nothing in it. Test the source itself before you commit a whole tub to it, or look up your municipal water report, which is published annually and gives hardness for your area.

Use a hose filter. Filtering on the way in removes chlorine from municipal supplies, heavy metals, sediment and trace contaminants before they ever reach the tub. It costs little, takes seconds to attach, and it reduces demand from the very first day rather than after the water has already been affected. If you buy one thing for water care, buy this.

Start with a clean tub. A fresh fill into a tub with residue on the surfaces is not a fresh start. Wipe the interior with a soft cloth and a mild cleanser, rinse thoroughly, and put in a clean filter if the tub has one.

Flush the hose. Run it for a minute before it goes in the tub. A hose that has been sitting in the sun holds warm stagnant water and whatever has grown in it.

Filling

Fill to the level your tub specifies. On a wood fired or hybrid tub that means the line marked on the tub, or at least 4 inches (10 cm) above the chimney collar, and the fire is never lit before the tub is full.

Two practical notes. Fill from the bottom where you can, since filling through a drain fitting rather than over the rim reduces aeration and gets the water moving less. And if you have a hot supply available, filling with warm water shortens heat-up time considerably and costs nothing extra.

Balancing a fresh fill

Always the same order, because each reading affects the ones after it.

Total alkalinity first, typically 80 to 120 ppm. It buffers pH, so correcting it first stops every later adjustment from being undone. Circulate fifteen to twenty minutes and retest before moving on
pH second, generally 7.4 to 7.6 depending on your tub. With alkalinity stable this usually needs less correction than people expect, and sometimes none at all
Calcium hardness third, typically 150 to 250 ppm. This is the one most owners never test and the one that matters most on a fresh fill from a soft source
Sanitizer last, once the first three are in range. Circulate and retest before anyone gets in. Never dose and step straight into the water

Add one treatment at a time, dissolved in a bucket of tub water first rather than tipped in dry. On a tub without a circulation pump, stir thoroughly with a paddle instead.

The most common first-fill mistake is adding sanitizer straight away because the water is new. Sanitizer in unbalanced water is far less effective, so you end up using more of it to less effect. More in why alkalinity holds pH steady.

What changes with your source

Municipal water

Usually closest to balanced, and usually the least work. Filter on the way in to remove the chlorine, and confirm hardness rather than assuming it, since some supplies run very soft.

Well water

Workable and common, and it varies more than any other source. Test hardness, pH and alkalinity on every new fill, and test for iron and copper as well, since both are common in well supplies and copper in particular is worth identifying before it goes into a tub. Use a hose filter as standard.

If your well water is very hard, dilute or blend sources rather than trying to soften it.

Rainwater

Free, soft, and the source that needs the most attention. It arrives with almost no calcium hardness, almost no alkalinity, and a pH on the acidic side, which makes it under-saturated and chemically aggressive until it is corrected.

Correct it on every fill rather than occasionally, because rainwater carries nothing over from the last one. Filter it on the way in, since roof-collected water carries organic matter, and expect to correct alkalinity more often if your collection surface sits under trees.

Water through a softener

Avoid it. A softener removes calcium as its entire function, which leaves you below range before anything else has happened. Find an unsoftened tap, or bypass the softener for the fill.

Lake, river and ocean

All workable with the right tub, and covered properly in handling water access at a remote property. Filter on the way in for sediment, and test before soaking as you would with any source.

The first week

New water moves more than settled water. Test two or three times in the first week rather than waiting for your normal schedule, since a fill that looked perfect on day one can drift by day three as everything finds its equilibrium.

Retest after any top-up too, particularly if the top-up came from a different source than the original fill.

If your tub is new

One thing that surprises new owners of cedar tubs: the first few fills will turn brown. That is tannins leaching from the wood, it is harmless, and it clears after roughly five full water changes.

The important part is that sanitizer waits until the water runs clear, because tannins in the presence of bromine can bond to the tub surface and stain it. Keep balancing alkalinity, pH and hardness throughout. More in cedar tannins and brown water.

If you have an AlumiTub

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, which is lower than the 3 to 5 ppm typical of a conventional spa.

Calcium hardness deserves particular care on a fresh fill, because water below roughly 100 ppm is under-saturated and chemically aggressive toward a marine-grade aluminum interior. Rainwater and softened water both land there, so both need correcting every time.

Use bromine rather than chlorine, and avoid salt chlorine generators and anything containing copper or silver. The Purifier hose filter in Good Clean Living Water Care is made for the fill step, and the range is built around these numbers so nothing needs checking against a label.

On a Wood Fired or Hybrid, fill to the line marked on the tub, or at least 4 inches (10 cm) above the chimney collar, and never light the fire before the tub is full.