First Chemicals After a Refill

The order is: total alkalinity, then pH, then calcium hardness if it needs it, then sanitizer, then shock. Several product labels will tell you to adjust pH before alkalinity. They are wrong for a reason worth understanding, and following them is why so many fresh fills refuse to settle.

Step 0: let it warm up, and test the raw water

Chemicals dissolve and react differently in cold water, and a reading taken at 60F will not match the same water at 100F. Let the tub reach temperature, then take a full test of the raw fill water before adding anything. That reading is the baseline everything else is relative to, and on well or softened supplies it is often surprising.

Step 1: total alkalinity, to 80-120 ppm

Alkalinity is the buffer that holds pH steady. Adjust pH while alkalinity is out of range and the pH drifts straight back, which is the loop most people spend their first fortnight in.

Add sodium bicarbonate at the label dose for your volume, jets running and cover off. Wait a full circulation cycle, then retest. Raise it in stages rather than in one dose, because bringing alkalinity back down takes acid, aeration and patience.

Step 2: pH, to 7.2-7.8

Now that the buffer is in place, the pH correction will hold. Most fill water sits high, so dry acid is usually the product. Add half the label dose, circulate a full cycle, retest, and repeat rather than dosing heavily once.^[source]

If pH is low and alkalinity was low too, do not reach for a pH increaser: raising alkalinity in step 1 usually brings pH up with it. The detail is on pH and total alkalinity.

Step 3: calcium hardness, only if the test says so

Target roughly 150 to 250 ppm in an acrylic spa.^[source] If your raw water is already in range, skip this entirely.

If it is low — common on softened supplies — raise it with calcium chloride to the bottom of the range, not the middle, because it will drift up on its own as you top up after evaporation. There is no calcium reducer, so an overshoot means a partial drain. The whole argument is on calcium hardness.

Step 4: establish the sanitizer

On bromine: add the sodium bromide bank first, then activate it with a dose of shock, then start the tablets in a floater. Skipping the bank is the classic first-fill mistake — tablets alone will not build a residual on fresh water, and you will spend a week wondering why the bromine reading stays at zero.

On chlorine: dose dichlor granules to the label for your volume, aiming for at least the 3 ppm free chlorine the CDC specifies for hot tubs.^[source] No bank step is needed.

If you are undecided, decide before the fill rather than after — bromine vs chlorine covers it, and switching later means draining again.

Step 5: shock, then wait and test

Shock the fresh fill with the jets running and the cover off for the time the label states. On bromine this also activates the bank. Then retest before anyone gets in, and test rather than relying on a clock: the wait time on the label is a guide, the reading is the answer. Details on shocking a hot tub.

The whole sequence, at a glance

Things that go wrong on a fresh fill

• Dosing cold water. Readings will chase you for days.
• Skipping the bromide bank. No residual, no matter how many tablets go in.
• Adjusting pH before alkalinity. The correction unwinds within days.
• Adding several products at once. You cannot tell what did what, and some combinations cloud the water.
• Assuming fill water is neutral. It carries whatever your supply carries, which on a well can be a great deal.
• Not retesting the next day. Fresh fills move as they settle. Check again after 24 hours.

The first week

Test daily for the first few days rather than weekly. A new fill takes a little time to stabilize, and small corrections made early are much easier than large ones made after the first weekend of use. After that, drop back to the weekly rhythm on the maintenance schedule.

The goal throughout is a residual you can hold consistently, which is what the CDC’s hot tub guidance is built around.^[source]