Why doesn't alkalinity self balance?

[Aaron Atkinson]

Hello,
Not sure if this has already been asked or not, but why doesn't alkalinity self-balance? My starting point is the fact that consumption increases & decreases with the starting alkalinity.

Assuming we are dosing above or below the actual demand within reasonable limits, the alkalinity will also raise or lower. Let's use over-dosing as an example.

If we over-dose the alkalinity, the baseline alkalinity should increase with a corresponding increase in demand. Thus, the balance point should be achieved at a higher baseline alkalinity. In my limited experience this doesn't occur. Why is that?

Thank you.

 

[wkscott]

Your premise that increased alkalinity will increase demand just doesn't work. More or fast growing hard corals on the other hand would increase demand thus lowering the alkalinity.

 

[Aaron Atkinson]

Thanks for the reply. I've been taking data on the subject, alkalinity demand vs. starting point, and in fact they are related. I have collected some data butt shown below and trend is there. This represents various data points over the course of a month or so. Randy (sorry to name drop) confirmed this matched expectations. This is what leads me to the question why doesn't alkalinity self balance?

 

[JimWelsh]

Have you been able to repeat this experiment?

 

[Aaron Atkinson]

Hello Jim. I only have one fish tank, so I guess I'm looking for volunteers.


Here is my original post a few days ago. Based on Randy's reply my data is accurate
https://www.reef2reef.com/threads/alkalinity-consumption-help.295763/

 

[Randy Holmes-Farley Verified Community Expert]

It is fully expected and very well demonstrated that increased alkalinity (and increased calcium and increased pH) increase the demand for alkalinity and calcium. This negative feedback effect on alkalinity dosing is a huge benefit to reefers because it makes maintaining a specific alkalinity a lot easier than if every little bit of extra alk cause alk to rise as much as you overdosed, and prevents it from falling as much as you underdose.

There are several reasons for this.

The first is that abiotic precipitation is driven by the supersaturation of calcium carbonate in the water (among other factors) and higher alk, pH, and calcium all increase the supersaturation and make precipitation more likely (or in other words, faster). This effect happens in all tanks, not just those with a perceived "problem" with precipitation. It happens on pumps and heaters (driven by temperature) and potentially on all other calcium carbonate surfaces (such as sand).

The second effect is that many hard corals are well known to hobbyists and in the scientific literature to calcify faster at elevated alkalinity. That is why, for example, Red Sea recommends high alkalinity when rapid growth of hard corals is the goal. Here's a scientific paper which also shows it:

http://www.aslo.org/lo/toc/vol_44/issue_3/0716.html
from it:

"The addition of 2 mM bicarbonate to aquaria containing tropical ocean water and branches of Porites porites caused a doubling of the skeletal growth rate of the coral."


Finally, this is easily recognized by most reefers by this significant fact: stop dosing alk for a few days or weeks and it NEVER drops to zero. Why? Because demand drops off fast as the alk drops below 6 dKH.

 

[JimWelsh]

My question about the repeatability of the experiment was not about the phenomenon itself, but rather the magnitude. I was surprised by how much demand increased relative to the starting alkalinity level. In the Porites experiment, it took an increase of 5.6 dKH to double the growth rate, but Aaron's data shows a doubling of the consumption with an increase of less than half that, from 1.1 dKH/day to 2.2 dKH/day with an increase in starting alk from 7.0 to 9.3.

 

[Aaron Atkinson]

Hello Jim. If it helps, I can tell you there has not really been any growth in the handful of LPS, soft coral frags or one SPS green pavona I have. I suppose this is mostly due to my wildly fluctuating alkalinity levels.... As a novice maintaining alkalinity has proven illusive so far, including what I'd describe as excessive white build up (limestone?) in the sump and frequent pump stoppages. This has led me to a "restart" in finding my alkalinity consumption --- eliminated kalkwasser from my ATO, stopped dosing for a few days, testing nightly on the kitchen counter much to my wifes displeasure.

As an engineer, I visualized recent and historical data to help me visualize whether my daily measurements were accurate and what would be the consumption rate at my target baseline alkalinity. What I believe so far is one of my data points is an outlier, which is helpful. So last night's data analysis led me to began dosing 1.6 dKH / day based on a target of 8.0 dKH alkalinity.

With respect to self-balancing, Randy do I understand you correctly that it will not self-balance higher due to precipitation? Is this the case with under-dosing demand?

 

[GoVols]

That is why, for example, Red Sea recommends high alkalinity when rapid growth of hard corals is the goal.

I've ran the Red Sea "Accelerated Growth Program" and it gave good results. Your SPS may brown out but when you go back to a normal alk dkh (When you get the growth you wanted) your SPS... will color back up.

 

[Randy Holmes-Farley Verified Community Expert]

With respect to self-balancing, Randy do I understand you correctly that it will not self-balance higher due to precipitation? Is this the case with under-dosing demand?

I'm not really sure what you mean by self balancing, but the demand rises as the alkalinity rises. If it didn't, it would be a nightmare of testing and dosing for reefers. Since it does, dosing amounts are less critical due to the feedback loop between alkalinity and the consumption of alkalinity. So slightly overdosing leads to slightly more demand, which partly offsets the overdosing.

How big the effect is may depend on too many factors to list them all, such as the pH, the magnesium, the calcium level, the surface area of calcium carbonate, availability of warm objects in the tank of various temps and dimensions, presence of organics that inhibit precipitation, etc.