August 27, 2026

Does your reef salt mix change your parameters?

You did a 20 percent water change on Sunday. Alkalinity read 8.3 dKH on Saturday and 9.1 on Monday, and you have not dosed anything. The kit is fine. You dosed the tank with a bucket of salt.

Here is the short version: yes, your reef salt changes your parameters, and by more than most people expect. The mainstream mixes do not make the same water. At the same salinity they land anywhere from about 7 to 12 dKH, roughly 400 to 500 ppm of calcium and about 1250 to 1450 ppm of magnesium. If the mix does not match the tank, every water change is a dose you did not measure. Sometimes that dose is useful. Sometimes it is why the corals were shut on Monday.

A water change is a dose

The shift from a water change is the fraction you changed, multiplied by the gap between the mix and the tank.

Take a tank at 8.0 dKH and a mix that lands at 12 dKH. A 20 percent change moves the tank by 0.2 x 4 = 0.8 dKH in the time it takes to refill. That is close to the ceiling of about 1 dKH per day most people treat as the safe rate of change for alkalinity. A 30 percent change with the same mix is 1.2 dKH in an hour, and in a low nutrient SPS tank that is enough for burnt tips and tissue loss over the following week.

It works the other way too. A tank dosed to 8.5 dKH, changed with a mix that lands at 7 dKH, loses 0.3 dKH per 20 percent change, and the dosing pump does not know. If alkalinity dips every Sunday and recovers by Wednesday, that is usually the whole explanation.

Calcium and magnesium move by the same rule but hide better. A 20 percent change with a mix at 465 ppm calcium into a tank at 420 ppm adds 9 ppm, below what a hobby calcium kit can resolve. The same change with a 1400 ppm magnesium mix into a 1300 ppm tank adds 20 ppm, also inside kit noise. Alkalinity is the one that shows on the test, but all three moved.

Over months, water changes pull an undosed tank towards the mix. A fish and softie system on a 12 dKH mix runs at 11 to 12 dKH indefinitely: stable, but at the top of the sensible range, and a mystery until you test the bucket. The full target chart, and which number to fix first, is in reef tank parameters.

Salt mix alkalinity, calcium and magnesium: what the label says

Every reef salt comparison comes down to the same three numbers, and they are the reef salt mix parameters worth writing down. These are labelled or commonly measured values at the salinity the manufacturer quotes. Treat them as starting points: formulations change and bucket to bucket variation is real.

Two families fall out of that table. The natural seawater style mixes (Tropic Marin Pro Reef, Aquaforest, Red Sea's blue bucket, Fritz RPM) land at 7 to 9 dKH, where a dosed reef normally runs, so a water change with them is close to neutral. The elevated mixes (Reef Crystals, Coral Pro, Fritz RPM Redline and the like) land at 10 to 13 dKH and are designed for the water change to be the dose. That suits an undosed tank with moderate demand; in a tank already dosed to 8.5 dKH they fight the doser.

Two things to check on the label:

• The salinity the numbers are quoted at. Aquaforest quotes at 33 ppt (about 1.025 SG), most others at 35 ppt (about 1.026 SG). Everything in the bucket scales together, so mixing to 35 ppt instead of 33 ppt raises alkalinity, calcium and magnesium by about 6 percent.
• Whether calcium is elevated with the alkalinity. Instant Ocean is the odd one: high alkalinity with calcium near 400 ppm. Used long enough on its own, a tank drifts to high alk and low calcium, exactly the pairing that precipitates.

Test a fresh batch before you trust it

The label tells you what the mix should make. Only a test tells you what this bucket makes.

Mix a batch from every new bucket or bag, bring it to your tank's salinity and temperature, and test alkalinity, calcium and magnesium. Write the results and the date on the lid with a marker.

Dry salt stratifies in shipping, so the first and last scoops from a bucket can mix to different water. Roll the sealed bucket end over end before opening it and stir the dry mix before you scoop. If the salt has gone clumpy or hard, moisture got in and some of the calcium and carbonate have already reacted into calcium carbonate that will not dissolve. That batch reads low on both, and the white haze that never clears is the precipitate.

Get salinity right first, because alkalinity, calcium and magnesium all scale with it: a batch mixed 10 percent thin reads 10 percent low on all three. Calibrate the refractometer with 35 ppt fluid, not RO water, and read the sample at room temperature. If you are not sure which salinity tester to believe, refractometer, hydrometer or digital covers what each gets wrong.

Match the mix to the tank, or the tank to the mix

Once you know what the bucket makes, there are three sensible options.

Pick a mix that matches how you run the tank. This is the honest answer to "what is the best reef salt". It is the one whose numbers sit close to your running numbers, that you can buy the same bucket of next month, and that you test. Dosing two part to 8 to 9 dKH? A salt that mixes to 7.5 to 9 dKH makes water changes nearly invisible on the chart. Running a heavy SPS tank at 9 to 10 dKH with no doser? An elevated mix can carry the load.

Adjust the batch, in the bin, never in the display. A mix that lands a little low is easy to correct. About 3 grams of sodium bicarbonate per 100 litres (26 gallons) raises alkalinity by roughly 1 dKH; about 7 grams of calcium chloride dihydrate per 100 litres raises calcium by about 20 ppm. Dissolve it in the circulating batch, wait an hour, re-test. A mix that lands high is harder, because you cannot take alkalinity back out. Blend it 50/50 with a lower mix (mixing brands is fine) or shrink the water change. If calcium is the number that keeps landing high, read how to lower calcium in a reef tank, because the fix is almost never removing calcium.

Move the tank towards the mix. If you like the salt but the tank runs 8 dKH and the mix makes 10, shift your target over a couple of weeks and trim the doser to suit, at no more than about 1 dKH per day and ideally half that. Once the tank sits at the mix's numbers, water changes stop being a dose at all.

Mixing saltwater for a reef tank: the routine

Most parameter surprises after a water change come from the mixing bin, not the bucket.

1. Start with RO/DI water at 0 TDS. Tap water brings its own alkalinity, calcium and silicate, none of it on the salt label.
2. Water first, then salt. Fill the bin, get a powerhead and heater running, then add salt to moving water. Never pour water onto a pile of salt: the pockets of brine at several times seawater strength precipitate calcium carbonate on the spot, and that calcium and alkalinity never come back. Roughly 35 grams per litre (about half a US cup per gallon) gets you near 35 ppt; weigh it, then trim with the refractometer.
3. Circulate until completely clear. For natural seawater style mixes a few hours is the minimum and overnight is fine. For elevated mixes read the bucket: Red Sea says no more than 4 hours of mixing for Coral Pro and use within 12, because the extra calcium and carbonate precipitate if left circulating. Do not aerate a batch (driving off CO2 raises pH and precipitation) and do not mix it hot, because calcium carbonate is less soluble in warm water.
4. Match temperature and salinity to the tank. Temperature within about 2 F (1 C). Salinity within 0.001 SG, roughly 1 ppt, of what the tank actually reads. Whether that should be 1.025 or 1.026 is answered in the ideal salinity for a reef tank. If the tank has drifted and you want to move it, do that no faster than about 0.001 SG per day, through the change water and top off, never by adding dry salt to the display.
5. Test alkalinity on the first batch from every bucket, and salinity on every batch. Log it next to the water change.
6. Add it where there is flow, into the sump or the return area, not over the rockwork.

The classic mistakes

Switching salts abruptly. A 25 percent change going from a 7.5 dKH mix to a 12 dKH mix is more than 1 dKH in an hour, plus a step in magnesium, calcium and whatever trace elements differ. Blend old and new over three or four water changes (25/75, then 50/50, then 75/25, then all new) and test the tank after each. If you have already done it and the corals have retracted, corals not opening after a water change walks through what to check and what to leave alone.

Mixing in the display or sump. Dry salt thrown into a running tank makes a plume of brine that kills whatever it settles on, drops undissolved grains onto corals, and precipitates calcium carbonate onto the sand and pumps. This includes topping up salinity with a handful of salt in the sump. Salinity moves through mixed saltwater, slowly, or it does not move.

Using a batch before it has finished mixing. A salt mix is a blend of separate compounds that dissolve at different speeds. The sodium chloride goes fast and gets the refractometer to 35 ppt while slower components are still on the bottom of the bin. Pour that and you have left calcium and carbonate in the bin. If the batch is still cloudy hours after everything should have dissolved, that is precipitate, not undissolved salt. It is usable but no longer the label, so test it.

Changing cold. A batch 4 F (2 C) below the tank is a common reason for corals to close for a day. It is not the salt, it is the thermometer you did not check.

Log the batch, not just the tank

The water change is the biggest single event in most tanks' weeks and the one people log least. Write the batch's alkalinity next to the tank's reading and you can see that the Sunday step is the salt, not the doser, or that the new bucket runs 0.6 dKH lower than the last. Without that note every step on the chart looks like a fault. To separate the water change from what the tank uses on its own, how to calculate daily alkalinity consumption covers the method.

Get the spreadsheet
Make a copy in Google Sheets (needs a Google account)
Download the Excel version (no account needed)

Free, no email, no attribution. Readings turn green inside your target band and red outside it, and the charts draw your target band as dashed lines.

Keeping a batch log and a tank log, and remembering which step on the chart was a new bucket, is exactly the kind of bookkeeping that quietly stops after a month. Recif is the reef companion app we built to take that over: it logs every test, charts each parameter against your own target ranges so a water change step stands out, works out consumption trends between changes, reminds you when a parameter has gone too long untested, and rolls the tank into one Stability Score. It is available on both the App Store and Google Play.