In most reef tanks, alkalinity drops because corals and coralline algae use it to build their skeletons, faster than water changes replace it. Nitrification, precipitation on equipment, a low-alkalinity salt mix, a failed doser and test error are the other usual causes. The fix is to measure how much alkalinity your tank uses per day and dose that amount every day, instead of correcting it after it falls.
Is my alkalinity actually dropping? #
Rule out the test before you change anything. Reagents age, especially once opened, and an old reagent is one of the most common reasons for a low alk reading. Retest with a second sample. If you can, test with a second method or a fresh reagent set.
Also consider normal variation. Hanna’s HI772 checker has a stated accuracy of ±0.3 dKH ±5% of the reading, and the Salifert kit reads in steps of about 0.3 dKH. A change of 0.3 dKH between two tests may be noise. A drop of 0.3 dKH every day for three days is a trend.
What causes alkalinity to drop in a reef tank? #
| Cause | How it shows | What to do |
|---|---|---|
| Coral and coralline growth | Steady decline, calcium falls too, faster as the tank fills in | Dose daily to match consumption |
| Nitrification | Alk falls, calcium doesn’t; often with heavy feeding or high bioload | Reduce input, and dose alk to match |
| Precipitation | White crust on heaters and pumps, cloudy water after dosing | Lower dose rate, check magnesium, avoid high pH |
| Low-alk salt mix | Alk drops right after water changes | Test fresh saltwater, and adjust or change salt |
| Doser failure | Sudden drop after steady readings | Check the container level, tubing and schedule |
| Calcium reactor change | Drift after adjusting CO2 or flow | Retune the reactor and test daily for a week |
Coral and coralline growth #
This is the reason in most tanks. Stony corals, clams and coralline algae pull calcium and carbonate from the water to build calcium carbonate. As they grow, they use more each month. A young reef may barely move its alkalinity in a week. A packed SPS system can use more than 1 dKH a day. Water changes replace some, but rarely enough once a tank has real growth.
Because skeletons are calcium carbonate, calcium drops alongside alk, at about 7 ppm of calcium per dKH of alkalinity. If both fall together at roughly that ratio, growth is almost certainly the reason. You’ll need to add calcium too; see how to raise calcium in a reef tank.
Nitrification #
Bacteria that convert ammonia to nitrate produce acid, and that acid consumes alkalinity. The University of Florida’s extension fact sheet on ammonia in aquatic systems notes that nitrification lowers pH unless enough alkalinity is present to buffer it. In a heavily fed tank, nitrification can account for a real share of alk loss, and it’s the one cause that lowers alk without lowering calcium.
Precipitation #
When alkalinity, calcium and pH are all high, calcium carbonate can form on its own on heaters, pump impellers and sand. Dosing too fast, or adding alk and calcium at the same spot, makes it worse. So does low magnesium, which normally slows precipitation. A tank that swallows larger and larger doses without the number rising is often precipitating them.
How do you measure daily alkalinity consumption? #
This is the step that turns constant correcting into steady dosing.
- Get alkalinity into range first. Raise it gradually if it’s low. Raising alkalinity with baking soda covers how.
- Keep everything else constant for a few days: same feeding, same lights, no water change.
- Test at the same time every day, before any dose, for three or four days.
- Calculate the average daily drop. If alk reads 9.0, 8.6, 8.1 and 7.7 over four mornings, the tank used 1.3 dKH over three days, about 0.43 dKH a day.
- Turn that into a daily dose. Multiply the daily drop by the amount of your supplement that raises your volume by 1 dKH.
- Dose it every day, split into several smaller doses if you can, or spread with a doser.
- Retest for a week and adjust. If alk still drifts down, increase the dose by about 10%. If it creeps up, reduce it.
Worked example: a 100-gallon tank using 0.45 dKH a day #
| Supplement | Amount for +1 dKH in 100 gal | Daily dose at 0.45 dKH/day |
|---|---|---|
| Dry baking soda | 11.4 g | About 5.1 g |
| Randy’s Recipe #2 (unbaked, 950 meq/L) | About 142 ml | About 64 ml |
| Red Sea Reef Foundation B (liquid) | About 38 ml | About 17 ml |
| ESV B-Ionic Alkalinity | About 48 ml | About 22 ml |
The per-dKH figures come from the makers’ stated rates and, for the DIY solution, Randy Holmes-Farley’s recipe strengths. Use the label on your own product, and base the math on net water volume.
Why does consumption keep changing? #
Because the tank keeps growing. A dose that held alkalinity steady in March may leave it falling by July as corals fill in. Remeasure consumption every month or two, and any time a trend appears in your readings.
Does low alkalinity cause other problems? #
Yes. Low alkalinity lowers the pH the tank can hold, so a slow alk decline often shows up as a pH decline too. Fixing alk first sometimes solves an apparent pH problem. If pH stays low with alk in range, see how to raise pH in a reef tank.
Swings also stress corals more than a steady low number. A tank that dips to 7 dKH before a weekly correction to 9 is on a sawtooth, and SPS in particular show it with tissue loss. Daily dosing matched to consumption removes the teeth.
How do you catch an alkalinity drop early? #
Testing more often during the measuring period, and keeping every result, is the whole method. See how often to test reef tank water for a routine.
Fish Tank Tracker makes the daily part quick. Its Full Test screen takes an alkalinity reading on its own (leave the other fields blank), and the chart plots each one against an 8-11 dKH band, so the slope of the decline is visible at a glance. The free version keeps a month of history, which covers a measuring period. Pro adds trend analysis that reports direction and a stability score for each parameter. Log in dKH (multiply meq/L by 2.8), and add dose changes in the notes so you can tie the chart to what you did.
Frequently asked questions #
How much alkalinity drop per day is normal? #
It depends on how much is growing. Tanks with mostly soft corals and fish may use very little, while a dense stony coral tank can use more than 1 dKH a day. Measure your own tank’s consumption rather than comparing with other people’s.
Why is my alkalinity dropping but calcium stays the same? #
Nitrification and other acid sources consume alkalinity without using calcium. So can test error on either kit. If alk falls while calcium holds steady for weeks, look at feeding and bioload, and retest calcium with a fresh kit.
Do water changes raise alkalinity? #
A water change raises alk if the new saltwater has higher alkalinity than the tank. Salt brands mix up to different alkalinity levels, so test a fresh batch to see what yours adds. In most growing reefs, water changes alone don’t keep up.
Should I dose alkalinity at night? #
Many reef keepers dose alkalinity at night or around the clock, since pH is lowest at night and alk dosing supports it. Spreading the dose across the day also keeps swings small.
Can alkalinity drop too fast? #
Yes. A sudden large drop points to a cause other than growth, such as precipitation after a dosing mistake, a doser failure or a test problem. Investigate right away rather than just dosing more.