The Ways Alkalinity can Accumulate in a Reef Tank

"Why is my alkalinity rising?" is a seemingly simple question that consistently pops up in the chemistry section of Reef2Reef. However, it is a complex question since there are numerous ways alkalinity can accumulate - some ways one may least expect.

The consensus of an alkalinity range in home aquaria is from 7dKH to 12dKH. Going below this range may halt calcification (growth) in organisms containing calcium carbonate; stress corals with the lack of carbonate & bicarbonate in the water; may suppress pH, which can dissolve coral skeletons below about a pH of 7.5. In contrast, very elevated alkalinity can increase the likelihood of precipitation, thus, wasting the alkalinity solution and may stress certain organisms, especially ones without adequate nutrients in the water.

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Sodium bicarbonate: a common alkalinity additive.

First and foremost, the way alkalinity can climb in a tank is through deliberate dosing. If you add an alkalinity supplement, but the level rises above your comfort, the obvious solution is to reduce the dose. The amount of solution dosing exceeds the demand for the tank. Some salt mixes, like Instant Ocean's Reef Crystals or Red Sea's Coral Pro Salt, contain higher alkalinity. Water changes may unintentionally exacerbate the issue despite helpful intentions.

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An RO/DI system can help prevent unintentionally adding alkalinity to a tank.

Another way alkalinity is rising is due to your source water. Depending on where you live, your tap water can have a significant amount of alkalinity. For example, tap water containing 10dKH and a salt mix containing 10dKH can result in 20dKH saltwater. Continuously topping off your evaporation with tap water may gradually increase the resulting alkalinity.
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Elevating alkalinity due to low demand is a great excuse to stock up on corals.

The next way unanticipated elevated alkalinity occurs is from low-demand tanks. It is natural for organics in the sand to have a low localized pH, which dissolves the calcium carbonate. In a well-stocked tank, the amount of calcium and alkalinity added is negligible and unnoticed; however, if there isn't much demand, there can be an apparent rise over time. We expect long-term reefers to replenish their sand bed from the dissolution of calcium carbonate because of this process.

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My DIY carbon dosing solution: made with 1,000mL Vinegar, 250mL Vodka, and 4tsp Glucose


Subsequently, the processes of denitrification will increase alkalinity. Reefers who use denitrification systems such as:​
  • Dosing vodka, vinegar, or sugar.​
  • Implementing a sulfur denitrator.​
  • Merely containing a deep sand bed.​
For every 50ppm of nitrates depleted, you will add 2.3dKH of alkalinity. Note that nitrates consumed from the nitrification process/ammonia dosing/foods will result in an alkalinity net of zero. Ammonia converting to nitrates depletes the same amount of alkalinity that rises when the denitrification process adds it back. Dosing of nitrates skips the alkalinity-depletion step.

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Beginners can fall prey to marketing schemes.

Additionally, a frustrating way alkalinity unexpectedly accumulates is from deceitful products. Whether mislabeling products is because of a lack of understanding of chemistry from the manufacturers or from an immoral method of boosting sales, it only hurts us, the consumers. Here are only a few examples of mislabeled products:​
  • Aquavitro's OH balance. Claims to elevate pH by using a hydroxide blend solution without boosting alkalinity; it's incorrect because any hydroxide solution will dissociate and use CO2 to make carbonate and bicarbonate, resulting in alkalinity.​
  • Aquavitro's "calcification" is a calcium additive made from calcium gluconate. Despite not stating much about its effect on alkalinity, bacteria will metabolize gluconate, adding alkalinity.​
  • Brightwell's Boost pH+. "Works with buffers and alkalinity products to increase pH to desirable 8,3, 8.4 or 8.5 levels without increasing alkalinity itself." This statement on the bottle is false. Any additive that raises pH will also raise alkalinity.​
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Throw out those expired test kits.

The final section will discuss how test errors and unique additives can impact alkalinity. The higher the salinity, the more abundant all the elements become. If you expect your salinity is around 35ppt, but in reality, it is higher due to a miscalibrated device, the alkalinity will be higher than expected (and so will all the other elements in the salt). Some alkalinity kits can test higher the older the regents become, so it is crucial to keep that in mind. The "unique" additives that may raise alkalinity are rocks made from cement, silica dosing (increases a trace amount), some amino acids, and other insignificant sources.

Although it can be frustrating to understand why your alkalinity is unintentionally rising, it can be satisfying once figured out. Elevated alkalinity can cause precipitation issues and can even have a snowball effect where fresh precipitation encourages more to form. Keeping alkalinity stable and in the optimal ranges make for happy and thriving organisms.

Happy Reefing! :)
 
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Miami Reef

Miami Reef

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"Elevating alkalinity due to low demand is a great excuse to stock up on corals."

Thanks @Miami Reef for provide much useful information and technical support for expanding my appreciation of coral husbandry. I think I can substitute your phrase quoted above to favorably convince the boss of the necessity of my binging. :)
Ha. I was so proud of that line. I was hoping someone would catch it. :D
 
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My 40 gallon is over a year old, no water changes, I have not yet had to dose alk thanks to carbon dosing. Corals are growing like mad.
Do you find you need to dose nitrates with carbon dosing?
 

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Do you find you need to dose nitrates with carbon dosing?
I haven't dosed nitrates, however I do occasionally dose some zeo products and not exactly sure what is in some of those. Typically nitrates are around zero but my phosphates are pretty high around .4 right now. But I have no complaints, the tank is looking better than I ever thought it would.
 

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