How to raise alkalinity in a reef tank
You tested tonight and got 6.5 dKH. Last time you looked it was somewhere around 8. The corals look mostly fine, except one Acropora with a pale patch at the base that you have been telling yourself is nothing.
Here is the short version: 6.5 dKH is not an emergency, and the fastest way to turn it into one is to fix it in a hurry. Natural seawater sits at about 7 dKH and plenty of healthy tanks run there on purpose. What damages corals is the speed of change. So bring alkalinity up by no more than about 1 dKH a day, 0.5 if you can stand the wait, re-test every day, and then work out why it fell so it does not fall again.
Most guides to how to raise alkalinity in reef tank water tell you the powder. Few tell you the pace.
What reef tank alkalinity should read
The mainstream target is 7 to 11 dKH. Most mixed reefs settle at 8 to 9. Tanks run very lean on nitrate and phosphate do better at 7 to 8, because high alkalinity with nothing to feed the tissue is what burns the tips off SPS. Where the number sits next to calcium and magnesium is laid out in the parameter chart.
Pick one number inside that band and hold it. Corals adapt to a level. They do not adapt to a level that moves 2 dKH a week.
Before you dose anything, confirm the reading. Hobby alkalinity kits carry 0.1 to 0.2 dKH of error, so run the test again from the same sample. If both say 6.5, believe it.
And be sure it is alkalinity you are worried about. A pH probe reading 7.8 at 7am is a different complaint with a different fix, and dosing alkalinity to chase it makes things worse. pH vs alkalinity sorts out which one is actually wrong.
What a low alkalinity reef tank does to corals
Alkalinity is, for practical purposes, the carbonate supply for building skeleton. When it thins out, calcification slows, and the fast growers feel it first.
At 6.5 dKH nothing dramatic happens in a day. Growth slows. Colours wash out a little over a couple of weeks. Soft corals and most LPS barely notice. Acropora and the other small polyp stony corals do, and if the level keeps sliding into the 5s you start to see tissue recede from the base or the tips, the pattern people call STN, slow tissue necrosis.
What triggers STN far more reliably than a low reading is a swing. A tank held at 6.5 for a fortnight and then pushed to 9 overnight loses more tissue than a tank left at 6.5 for a month. Low and steady is a problem you solve this week. Low, then high, then low again is a problem you cannot solve.
How fast to raise dKH in a reef tank
Search "raise dKH reef tank" and the answers run from "a point a day" to "whatever you like". The ceiling is about 1 dKH per 24 hours. The comfortable pace is 0.5 dKH per day, split into two or more doses, or dripped in over the day by a dosing pump.
From 6.5 to 8.5 is a 2 dKH climb: two days at the ceiling, four at the comfortable pace. Four days, not one evening.
There is a catch that trips up almost everyone. While you are correcting, the tank is still consuming. If your corals use 0.6 dKH a day and you add 0.5, you will test on day four and find you have gone backwards. The correction sits on top of the maintenance dose, not instead of it. If you do not know your daily consumption, measuring it takes four or five days of readings and the consumption calculator does the arithmetic.
Then the rule that makes all of this safe: test every day, same hour, before that day's dose. If you overshoot, stop dosing and let consumption bring it back down.
Two rules that never bend. Dissolve every dry product in RO/DI water first, and dose into high flow, at the sump return or in front of a powerhead, never into a quiet corner of the display where a concentrated slug of carbonate sits against a coral. And keep alkalinity and calcium doses at least 30 minutes apart and in different places, because together in concentrated form they precipitate as calcium carbonate and you lose both.
Baking soda for reef tank alkalinity
The baking soda reef tank alkalinity trick is real, and it is the cheapest supplement there is: the box of bicarbonate of soda in the kitchen cupboard is the same sodium bicarbonate sold in reef branded tubs, without the mark up. Check the label says sodium bicarbonate only; baking powder is a different product.
About 30 mg of sodium bicarbonate raises 1 litre by 1 dKH, which is 3 g per 100 litres, or roughly 11 g per 100 US gallons. Weigh it on a kitchen scale that reads to 0.1 g.
Worked through for a system holding 200 litres (53 gallons) of actual water, after subtracting rock and sand:
- 1 dKH needs 200 × 30 mg = 6 g of sodium bicarbonate
- A 0.5 dKH daily step needs 3 g
- Dissolve the 3 g in 250 mL of RO/DI water, add it slowly into the sump return, and repeat for four days
- Add the maintenance dose on top, every day, at the same time
The pH question. Bicarbonate nudges pH slightly down for a few hours when it goes in, then drifts back as the water re-equilibrates with the air, so its net effect on pH is close to zero. That makes it the right choice for a tank whose pH already runs high, 8.3 and above.
If your pH runs low, below about 7.9 in the morning, use sodium carbonate instead, sold as soda ash. It supplies the same alkalinity and raises pH while it does it. You can make it from baking soda: spread it on a baking tray and bake at 150 to 200 C (300 to 400 F) for an hour. The powder loses about a third of its weight as water and CO2, and what is left is sodium carbonate. Each gram now carries more alkalinity, so the dose changes: about 19 mg per litre per dKH, which is 1.9 g per 100 litres, or 7 g per 100 US gallons. Weigh it after baking, not before.
Soda ash solution is strongly alkaline, so it spikes pH where it enters the water and will precipitate calcium carbonate as a white cloud if it goes in fast or into slack flow. Drip it, and if the water clouds, stop.
Two-part, kalkwasser, calcium reactor and all-in-one products
Baking soda fixes the number. The products below are how you keep it fixed, and each has one thing it is bad at.
Two-part. Separate bottles of alkalinity (sodium carbonate or bicarbonate) and calcium (calcium chloride), sold by ESV, BRS, Red Sea, Tropic Marin and others. The label gives a millilitres per dKH figure for your volume, and the dosing calculator post walks through converting it. For a correction, dose the alkalinity part only until you reach target, then dose both parts daily. Separate outlets, 30 minutes apart.
Kalkwasser. Calcium hydroxide dissolved in RO/DI water; about 1.7 g per litre is all that dissolves. The clear liquid has a pH above 12, so it is never poured in, only dripped or dosed slowly through the top up line, ideally overnight when tank pH is lowest. It supplies alkalinity and calcium in exactly the ratio corals consume them, and it raises pH. The limit is volume: one litre of saturated kalkwasser in 100 litres of tank water adds about 1.3 dKH, and you can only add as much as the tank evaporates, so it maintains a moderate demand and cannot correct a low reading. Fix the number with bicarbonate first, then let kalkwasser hold it.
Calcium reactor. Aragonite media dissolved by CO2, feeding balanced alkalinity and calcium back to the tank. Excellent for heavy SPS demand, and it lowers pH, so it usually gets paired with kalkwasser or a CO2 scrubber. It takes days to tune and responds slowly, which makes it the wrong tool for a correction.
All-in-one products. Tropic Marin All-For-Reef, the Balling style kits from Fauna Marin and Aquaforest, and similar. They deliver alkalinity, calcium and magnesium in a fixed, balanced ratio, and that fixed ratio is the catch. If alkalinity is at 6.5 and calcium is already at 450 ppm, an all-in-one cannot raise one without pushing the other. Correct with plain bicarbonate, then go back to it for maintenance.
Why alkalinity and calcium have to rise together
Calcification takes calcium and carbonate in a fixed ratio: every 1 dKH of alkalinity consumed goes with about 7 ppm of calcium.
It means two different jobs get confused. A correction fixes whichever one is off. A maintenance dose replaces both, in ratio, every day. If alkalinity is 6.5 and calcium is 420 ppm, the correction is alkalinity only. Adding calcium you do not need just parks it at 480 ppm.
The reverse mistake is more common. Someone tests calcium at 500 ppm and alkalinity at 6.5, decides calcium is the problem, and starts water changes to bring it down. Calcium is high because it was not being consumed at the rate alkalinity was, usually because alkalinity was the thing limiting growth. Raise alkalinity, and calcium falls into line over the following weeks as calcification catches up. Lowering calcium directly is almost never the answer.
Once both are on target they leave together, so they have to be replaced together. Dose alkalinity daily and calcium never, and calcium drifts down at 7 ppm per dKH, invisible day to day and obvious after a month.
Magnesium is the third leg. At 1250 to 1350 ppm it slows calcium carbonate from precipitating out of solution. Let it fall to 1100 and alkalinity becomes hard to hold, because what you add comes straight back out on the pump and heater as white crust. If you correct alkalinity twice and it will not stay up, test magnesium before you test your patience.
Why it dropped, and holding it there
Getting to 8.5 is the easy part. Staying there is what separates a stable tank from one that lives on corrections. The usual reasons a tank arrives at 6.5:
- Consumption outgrew the dose. Corals grew, coralline took off, demand went up and the dose did not. The most common by far; re-measure consumption every four to six weeks.
- The doser stopped, or the container ran dry. Check the tube, the level, and that the pump ran today.
- A water change with low alkalinity salt. Some salt mixes come out near 7 dKH. A 25 percent change with one of those pulls a tank at 8.5 down by 0.3 to 0.5 dKH. Test the new water before it goes in.
- Low salinity. Salt mixed to 1.022 SG (29 ppt) instead of 1.026 (35 ppt) carries about 17 percent less of everything, alkalinity included.
Then the routine is simple: know your daily consumption, dose it in as many small doses as your equipment allows, and test daily if you dose, twice a week if you do not.
A four day correction produces a dozen readings and doses. Keep them, because the next time alkalinity slides you will want to know how fast it slid and what fixed it.
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.
The short version
- Target 7 to 11 dKH, most tanks 8 to 9, pick one number and hold it
- Raise no more than 1 dKH per day, ideally 0.5, on top of your maintenance dose
- Sodium bicarbonate at 30 mg per litre per dKH, or soda ash at 19 mg if pH runs low
- Test daily at the same hour, stop when you hit target
- Correct alkalinity alone, then maintain alkalinity and calcium together, and check magnesium if it will not hold
A four day correction with a maintenance dose on top is a dozen readings, a dozen doses and a lot of arithmetic to keep straight on paper. Recif is the reef companion app we built to carry that: it logs every test, charts each parameter against your own target ranges so a climb that is too steep is visible before it becomes a swing, works out consumption trends from your readings, reminds you when a parameter has gone too long untested, and rolls the whole tank into one Stability Score. It is available on both the App Store and Google Play.