How to raise pH in a reef tank
Your probe reads 7.85 before the lights come on and never quite makes 8.1 by evening. Alkalinity is fine, the corals look fine, but the number nags, and every thread you open says you should be at 8.3.
Here is the short version: low pH in a reef tank is almost always the air in your house, not the water in your tank. The fixes that work all deal with carbon dioxide. The products sold to raise pH mostly raise alkalinity instead, and that is the mistake that turns a cosmetic problem into a real one.
Why reef tank pH runs low
Seawater pH is set by two things: how much alkalinity is in the water and how much carbon dioxide is dissolved in it. Alkalinity you control. CO2 you mostly do not, because the water constantly exchanges gas with the air above it, and your skimmer pumps that air through the tank all day.
Outdoor air is about 420 ppm CO2. Seawater at 8 dKH in equilibrium with outdoor air settles around pH 8.2. Indoor air in a closed house is commonly 600 to 1500 ppm, and past 2000 in a shut bedroom overnight or a small room with a gas cooker. Each doubling of CO2 costs roughly 0.3 pH units. So the same 8 dKH water under 1000 ppm of room air settles near 7.8, and nothing is wrong with the water. It is telling you what the room is doing.
That is why the classic low pH reef tank is in a sealed house in winter, and why two identical tanks in two houses can read 7.8 and 8.3.
The tank makes some CO2 of its own (respiration, calcium reactor effluent, carbon dosing), but on most systems the room dominates.
pH also swings 0.15 to 0.4 units every day, up through the photoperiod as photosynthesis pulls CO2 out of the water and down overnight. Comparing a pre dawn low to a late evening high is how people invent problems. Test at the same hour.
What range is actually fine
The commonly quoted range for a reef tank is 7.8 to 8.4, and the target most experienced keepers aim at is 8.1 to 8.3 during the day. Corals calcify more easily at the top of that band than at the bottom, which is why SPS keepers chase it. Below about 7.7, calcification gets hard and aragonite sand can start to dissolve. Above 8.5, calcium carbonate starts precipitating onto heaters and pump impellers, and alkalinity and calcium begin to fall for no reason you can find.
So a tank sitting at 7.9 with healthy corals is not an emergency. Raising it is worth doing, but it is a tuning job, and nothing below should be done in a hurry.
First, prove the low reading is real
A drifted probe is a fake low pH, and the most common false alarm in this topic.
Any pH probe in a reef tank drifts with age, and reading low is the usual complaint. Six months without calibration can be worth 0.1 to 0.2 units; an eighteen month old probe can read 0.3 low and still look healthy on the controller. Open any pH probe reef tank thread and you will find someone who bought a CO2 scrubber to fix a probe.
Calibrate with fresh pH 7 and pH 10 buffers, not 4 and 7, because a reef probe needs a reference point above the range it reads. Monthly if the probe controls anything (a kalk doser, a calcium reactor solenoid), every month or two if it only displays a number. Keep the buffer bottles capped, since pH 10 buffer absorbs CO2 and drifts low itself once opened, rinse the probe in RO between solutions, and never let the bulb dry out. Replace it when it will not hold calibration for more than a few weeks; most hobby probes are done between 12 and 24 months.
Then test alkalinity. If alkalinity is 6 dKH, your pH is low because alkalinity is low, and the fix is the raise alkalinity job, not any of the CO2 methods below. If alkalinity is 8 to 9 dKH and pH is still under 8.0, you have a CO2 problem. The two get confused constantly, and pH vs alkalinity walks through which one is actually wrong.
The one hour cup test
Take a cup of tank water, measure its pH, run an airstone in it for an hour with outside air (air pump on the windowsill), and measure again. If pH rose by 0.1 or more, the water holds more CO2 than it would in equilibrium with outside air. Now repeat with a fresh cup and indoor air. If pH rises indoors too, the tank is under aerated and more surface agitation or a bigger skimmer will help. If it only rises with outside air, the room is the problem, and no amount of indoor aeration will fix it.
Most people land in the second group. The fixes below are in order of how much they typically move the number. Expect 0.2 to 0.4 units from the whole list; most tanks reach 8.1 to 8.3 with the first two alone, and 8.5 is not a target.
Fix 1: fresh air
Cheapest, and often the biggest single gain. Open a window in the tank room and watch the probe for a day. In a sealed house pH climbs 0.1 to 0.2 within hours, and the size of that jump tells you how bad your indoor air is.
The permanent version is to feed the skimmer outside air: a length of airline from the skimmer's air intake to a window vent, an attic, a crawl space or a wall grille. Use the widest tubing that fits, because a long run of 4 mm airline strangles the skimmer's air draw and you lose both skimming and pH. Put a foam plug on the outdoor end to keep out insects and rain. Expect 0.1 to 0.3 units.
Fix 2: a CO2 scrubber on the skimmer intake
Search for a CO2 scrubber reef tank setup and you will find pages of them, for good reason: it is the fresh air fix for people who cannot open a window.
It is a canister of soda lime media (calcium hydroxide with a little sodium hydroxide) on the skimmer's air intake. Every litre of air the skimmer draws passes through the media and comes out with its CO2 stripped to near zero, so the tank equilibrates with clean air instead of room air. Typical gains are 0.2 to 0.3 units, sometimes more in a bad room, and it shows within a day.
The media changes colour as it exhausts (white to violet in most brands) and lasts anywhere from a few days on a big skimmer in a stuffy house to a month on a nano. Exhausted media reverts to white if it sits, so it can look fresh and be dead; go by the pH, not the colour. Media dust is caustic, so keep a foam pre filter between the media and the skimmer. When it runs out, nothing bad happens; the pH drifts back to where it was.
Fix 3: kalkwasser
Kalkwasser (limewater) is calcium hydroxide dissolved in RO water. Saturated, it holds about 800 ppm calcium and around 40 meq/L (roughly 114 dKH) of alkalinity at a pH of about 12.5, and every drop consumes CO2 while adding calcium and alkalinity in the ratio corals use them.
That is why the kalkwasser pH lift is permanent rather than a spike: a tank that gets its 1 dKH a day as kalk instead of two part runs 0.1 to 0.2 units higher, all the time, because the dosing itself removes CO2. Two part leaves pH roughly where it was; a calcium reactor adds CO2 and pulls it down.
Mix it at about 2 level teaspoons per gallon (roughly 2 g per litre; the excess that will not dissolve settles, and you draw from the clear liquid) and keep the container sealed, because kalk pulls CO2 out of the air and turns into useless calcium carbonate.
Now the warnings, because this method can kill things.
Never dump kalkwasser. A slug of pH 12.5 water burns whatever it lands on and precipitates a cloud of calcium carbonate around it, and 100 mL (about 3 fl oz) is enough to do damage in a nano. It goes in by slow drip or dosing pump, into a high flow area of the sump, never into the display, and never anywhere without flow. Dose overnight, when pH is at its low and the lift is most useful.
Kalkwasser adds alkalinity, so it is capped by consumption. Every 1% of tank volume of saturated kalk adds about 1.1 dKH and 8 ppm calcium, so on a 200 litre (53 gallon) tank, 2 litres a day is 1.1 dKH. If the tank only consumes 0.5 dKH a day, that dose sends alkalinity climbing 0.6 dKH every day, and alkalinity should never rise faster than about 1 dKH per day. Work out your daily alkalinity consumption first, cut your two part or reactor output by whatever the kalk now supplies, and test alkalinity daily for the first week.
The safest way to run it is through the auto top off, which only replaces evaporation, so you physically cannot add more than you evaporate. Kalk adds no magnesium, so test that monthly, and if pH ever creeps over 8.5, particularly with a scrubber running as well, back the kalk off.
Fix 4: light the refugium at night
Run the refugium light on the opposite schedule to the display, so that while the tank is dark and respiring, the chaeto is photosynthesising and pulling CO2 out of the water.
This does not raise the daytime high. It fills in the overnight dip, which is usually the reading that alarmed you, by a modest 0.05 to 0.15 units, with nutrient export as a bonus.
Why pH buffers raise alkalinity instead
This is the classic mistake, and most of what comes up when you search how to raise pH in reef tank forums leads straight into it, because every "pH up" and "reef buffer" product on the shelf is sodium carbonate, sodium bicarbonate, or a blend.
Dose one and pH jumps 0.1 to 0.2 units for an hour or two, then falls back as the water re equilibrates with the same CO2 rich room air. The pH went nowhere. What stayed behind is alkalinity. Do that daily and a tank at 8 dKH is at 12 dKH inside two weeks, at which point calcium carbonate starts precipitating, SPS tips burn, and you have a real chemistry problem instead of a cosmetic one.
The numbers do not work either. At a fixed CO2 level, doubling alkalinity raises pH by only about 0.3 units, so pushing 8 dKH to 12 dKH buys roughly 0.18 at the cost of running alkalinity 50% high. Fresh air on the skimmer gets you more for nothing.
The one case where alkalinity is the right lever is when it is genuinely low. Under 7 dKH, bring it back to 8 to 9 dKH and pH follows. If you have to dose alkalinity anyway, soda ash (sodium carbonate) gives a small pH lift with each dose and baking soda (sodium bicarbonate) gives none or a slight drop, so pick soda ash. The reef tank parameters chart covers which number to fix first when several look off at once.
Log pH at the same hour, or you will not know what worked
Every fix above takes a day or more to show, and every one is confounded by the daily swing. Open a window at 9am, read 8.15 at 6pm, and you have learned nothing, because 6pm was always going to be higher than 9am. Same hour, day after day, before and after.
Log alkalinity beside it, because two of the fixes move alkalinity too. If kalk is creeping your dKH up 0.3 a day, you want to see that on day three, not on day ten when the tips go white.
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.
Chasing pH means watching two numbers move slowly across weeks, at one fixed hour, while you change one thing at a time, and that is the kind of bookkeeping a notebook loses on day four. Recif is the reef companion app we built to handle it: it logs every test, charts each parameter against your own target ranges so a 0.1 unit gain is visible, works out consumption trends so you can see what the kalk is doing to alkalinity, 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.