The first step is to test your water to determine the current degree of carbonate hardness, or dKH for short. The second step is to perform a minimum 25% water change and continue water changes until achieving the desired dKH. Once your dKH is 12 or less, you should be able to rely on your corals to use the alkalinity in the water naturally. Use a salt mix or pre-mixed saltwater with alkalinity less than the tank’s current dKH to dilute the alkalinity. Do not use organic acid as its effect is temporary.
Alkalinity in reef tanks
Alkalinity is measured in “degree of carbonate hardness” or dKH for short, and natural seawater has a dKH of around 8 or 9. Alkalinity is essential to the calcification of corals in a reef tank, and the ideal range for alkalinity in home reef aquariums is between 125-200ppm of calcium carbonate (7-11 dKH). Maintaining alkalinity above 11 dKH can lead to alkalinity burn and precipitation of calcium carbonate.
Alkalinity Testing
The first thing to do is verify that you have a high alkalinity issue. Routine water testing is the best way to keep yourself and your reef tank inhabitants from having parameter issues in the first place.
The best alkalinity (dKH) testing method uses digital Hanna Checker colorimeters. The digital readout makes it a simple and foolproof process.
Alkalinity is the ability of a system to buffer the pH as acids are introduced. In saltwater systems, alkalinity is present in two primary forms; carbonate (CO32-) and bicarbonate (HCO3-). Alkalinity is a critical parameter for reef aquariums where drastic pH swings can be catastrophic to corals.
Further, alkalinity is essential to the calcification of corals. The ideal range for alkalinity in reef aquariums is between 125-200ppm of calcium carbonate (7-11 dKH). Many test kits on the market rely on a color change that needs to be judged by the eye. The HI772 from Hanna makes alkalinity testing easy by simply mixing the sample with a reagent and pressing a button.
Once you have verified that you have an alkalinity issue, you can move on to the methods outlined below to bring your reef tank alkalinity back to a manageable level.
What happens if alkalinity is too high?
Three things that can happen in a reef tank when your alkalinity is too high:
- Alkalinity Burn: Alkalinity burn is the visible loss of tissue at the coral’s tip or edges. Anecdotal evidence indicates that this condition is from increased photosynthesis, creating accelerated skeletal growth that the thin coral tissue can’t match. Alkalinity burn usually is worst in higher PAR areas of the tank.
- Precipitation of calcium carbonate: Alkalinity and calcium are directly linked in reef tank water chemistry. If alkalinity is too high, you can get a swift drop in calcium. This alkalinity and calcium imbalance will lead to an unbalanced ratio requiring frequent testing and dosing to restore the balance. I will address how to balance alkalinity and calcium later in this post.
- Coral and Coralline algae growth: See, it’s not all bad! Managing your alkalinity levels toward the high side of 10-12 dKH can spur coral and coralline algae growth.
How to lower alkalinity in reef tanks – two easy methods:
Water Changes
Perform a minimum of 25% water changes daily until your alkalinity reading is within your desired range (typically 8-12 dKH.) Use a salt mix with a low dKH of 8 or less to dilute your alkalinity levels the fastest.
It might not be the most exciting way to lower alkalinity in your reef tank, but it works! You also get all of the other side benefits of water changes.
Simply Wait!
- Your corals will uptake alkalinity daily, causing your dKH to drop over time.
- Waiting works best for alkalinity levels not too far above the recommended range.
- Depending on the number and size of your stony corals, your dKH could drop as much as 1-2 dKH per day.
- This method will not negatively affect the pH of your tank.
Do NOT Use Organic Acid
Adding an organic acid, such as vinegar will only have a limited short-term effect on your reef tank’s alkalinity reading. Potentially only hours.
The effectiveness of organic acid is an internet myth that even nanoreefadviser.com was once guilty of propagating. According to Randy Holmes-Farley* ‘Any impact on alkalinity only lasts until the acetate is metabolized by bacteria to convert it into CO2 and bicarbonate.’
*Randy Holmes–Farley has a BA in chemistry and biology from Cornell University (1982) and a Ph.D. in chemistry from Harvard University (1986)
Solving Calcium And Alkalinity Problems
I’ve found that you can have alkalinity and calcium balance issues in one of four categories. If you want to get deep into the science of how alkalinity, calcium, and magnesium interact, see scientific source information here.
In summary, below, I have included how to deal with each of these conditions.
Corrections to Alkalinity and Calcium Balance Issues
Calcium and alkalinity are too high: The problems may correct themselves over time as calcium and alkalinity are used equally.
Alkalinity and calcium are too low: You need to add equal portions of a calcium and alkalinity supplement. If the problem is not easily corrected, test your magnesium. It might be your magnesium readings are below 1200 ppm. Add magnesium if required.
Calcium is too high relative to Alkalinity: You need to increase your alkalinity supplement. Do not add kalkwasser or other calcium-balanced additives, as this will only continue to raise calcium—instead, only dose alkalinity or the alkalinity portion of a two-part supplement. If the problem is persistent and not easily corrected, test your magnesium. It might be your magnesium readings are below 1200 ppm. Add magnesium if required.
Kalkwasser is the German term for limewater, a saturated solution of calcium hydroxide in water. Calcium hydroxide—Ca(OH)2—is sold in a powdered form for aquarium use.
Nano Reef Adviser
Alkalinity is too high relative to Calcium: You need to increase your calcium supplement. Do not add kalkwasser or other calcium-balanced additives, as this will only continue to raise alkalinity. Instead, only add calcium or the calcium portion of a two-part supplement. If the problem is not easily corrected, test your magnesium. It might be your magnesium readings are below 1200 ppm. Add magnesium if required.
I have another post on adjusting alkalinity and pH with baking soda (sodium bicarbonate) and soda ash (sodium carbonate.) Results are instant – and it’s a great way to make significant but safe adjustments. See my post here.
What Is Alkalinity?
Alkalinity measures how much a given water sample can resist the pH-dropping effects of acid. The more acid required to cause that chemical change, the more resistant the solution is to the chemical change. This is desirable when trying to grow sensitive organisms such as coral.
In addition to chemical stability, alkalinity is essential for stony coral growth because corals and other organisms use alkalinity over time. Stony corals include large polyp stony coral (LPS) and small polyp stony coral (SPS.) These hard corals use alkalinity, calcium, and other elements to form support structures like exoskeletons.
Alkalinity is measured in “degree of carbonate hardness” or dKH for short, and natural seawater has a dKH of around 8 or 9.
What is the ideal alkalinity for reef tanks?
Alkalinity for reef tanks should range from 8 to 12 dKH. It has been suggested that some corals grow at an increased rate when alkalinity is stable at the highest range.
Being at the high or low side of any range means you can miss the mark and fall out of the goal range. Most reefers pick a dKH somewhere in the middle and focus on stability. For many reefers, that choice is frequently around 9 to be closer to natural seawater. Here’s a video discussing elevated alkalinity and other elements:
Additionally, maintaining your water parameters where they are consistently stable over time is a key to reef tank success.
Stability can be achieved by using a quality salt mix at your target alkalinity and by dosing alkalinity along with other vital elements such as calcium and magnesium.
I have a chart of salt mixes and their targets for the three major elements below. Since we started this post anticipating that we need to know how to lower alkalinity in reef tanks, I listed the salts with the lower alkalinity at the top of the chart.
Chart of Salt Mixes
| Salt Product | Calcium | Alkalinity | Magnesium |
| Marine Environment | 480 | 7.5 | 1450 |
| Aquatic Gardens | 430 | 8 | 1240 |
| Red Sea | 400 | 8 | 1300 |
| Oceanic | 580 | 8.5 | 1650 |
| Tropic Marin Pro Reef | 450 | 8.5 | 1380 |
| CoraLife | 560 | 9 | 1380 |
| Crystal Sea Marinemix | 340 | 9 | 1050 |
| Crystal Sea Marinemix Bioassay | 340 | 9 | 1050 |
| Tunze Reef Salt | 420 | 9.5 | 1350 |
| SeaChem aquavitro salinity | 422 | 9.8 | 1336 |
| D-D H2Ocean | 450 | 10 | 1380 |
| OceanPure | 510 | 10 | 1320 |
| SeaChem Marine Salt | 500 | 10 | 1400 |
| SeaChem Reef Salt | 540 | 10 | 1450 |
| Tropic Marin | 375 | 10 | 1230 |
| Brightwell Neomarine | 370 | 11 | 1140 |
| Instant Ocean | 400 | 11 | 1350 |
| Kent | 540 | 11 | 1200 |
| Reefer’s Best | 420 | 11 | 1200 |
| Red Sea Coral Pro | 470 | 12.5 | 1400 |
| Reef Crystals | 490 | 13 | 1440 |
Summary
How to lower alkalinity in a reef tank
Managing alkalinity is critical to success with a reef aquarium, and if you find your reef tank parameters are out of whack – it’s time to take action.
There are two easy steps to lower alkalinity in reef tanks:
- Water changes: Perform 20-25% water changes daily until your alkalinity reading is within your desired range (typically 8-12 dKH.)
- Wait: Your corals will uptake alkalinity daily, causing your dKH to drop over time. Waiting works best for alkalinity levels not too far above the recommended range.
Solving Calcium And Alkalinity Problems
I’ve found that you can have alkalinity and calcium balance problems in one of four categories. I have included how to deal with each issue in the summary below.
Issues and Corrections
- Alkalinity and calcium are too high: The problems may correct themselves over time as calcium and alkalinity are consumed equally.
- Calcium and alkalinity are too low: You need to add equal portions of a calcium and alkalinity supplement.
- Calcium is too high relative to Alkalinity: You need to increase your alkalinity supplement.
- Alkalinity is too high relative to Calcium: You need to increase your calcium supplement.
Alkalinity in a reef tank
Alkalinity measures how much a given water sample can resist the pH-dropping effects of acid. The more acid required to cause that chemical change, the more resistant the solution is to the chemical change.
This resistance to change (stability) is desirable when growing sensitive organisms such as coral.
Alkalinity is measured in “degree of carbonate hardness” or dKH for short, and natural seawater has a dKH of around 8 or 9.
Ideal alkalinity for reef tanks
What should alkalinity be in a reef tank? Alkalinity for reef tanks should range from 8 to 12 dKH.
It has been suggested anecdotally that some corals grow at an increased rate when alkalinity is stable at or near the highest dKH of that range.
Nano Reef Adviser
Maintaining your water parameters consistently stable over time is key to reef tank success. Stability can be achieved by using a quality salt mix at your target alkalinity and by dosing alkalinity along with other vital elements such as calcium and magnesium.
You will want to dose alkalinity, calcium, and other elements as consumed by your tank—dose based on the pace of your coral’s uptake of each component.
Additionally, I have another post on adjusting alkalinity and pH with baking soda (sodium bicarbonate) and soda ash (sodium carbonate.) The results are instant – and it’s a great way to make significant but safe adjustments. Check it out here.
How do you manage alkalinity, calcium, and magnesium in your reef tank? Do you have any tips or tricks on how to lower alkalinity in your reef tank? Leave a comment below, and happy reefing!