Can't figure out what's eating up my ALK

Creggers · Mar 23, 2023

Hello R2R,

I come with a strange situation that me nor my local reef friends can figure out. My new tank has been up and running for 3 months or so - at this point it's lightly stocked on both the Fish and Coral front.

My ALK keeps on falling and falling despite adding around 300ml of concentrated aquaforest. I just recently switched to the concentrated formula after I realized that my tank was rapidly eating the alk.

I'm attaching a screen shot of my levels - last night I manually dosed in alk to get the level up to ~9.6 or so, and my most recent test (not even 24 hours later) read out at 8.5. (hannah confirmed)

My Calcium is 500 and above what I'd normally be running it at thanks to swapping over to the pro formula of aquaforest - I'm slowly letting that come down. MG is hovering around 1400 which is my target - daily swings of around 200 or so is expected.

I'm including a screen shot of my apex below so you can see for yourselves. It's a real head scratcher - my coral are all thriving and I'm even seeing new growth out of my acros - but long term this can't continue happening.

I'm going to do a triton test as soon as these levels settle around my goals but until that happens I'm holding off.

This is an assumption on my part but the only thing that makes a little sense to me is my substrate / rocks are absorbing it in some way... I have florida crushed coral on top of the ocean direct sand with Caribsea rocks... I don't logically believe that but I can't figure it out!

PH hovers between 8 and 8.5 which implies that CO2 isn't an issue.

Let me know if there's anything else missing <3

Randy Holmes-Farley · Mar 23, 2023

Creggers said:
My Calcium is 500 and above what I'd normally be running it at thanks to swapping over to the pro formula of aquaforest - I'm slowly letting that come down. MG is hovering around 1400 which is my target - daily swings of around 200 or so is expected.

That change in magnesium is test error or a large salinity change. Magnesium NEVER moves up and down 200 ppm unless it is a big salinity change.

Excess CO2 is never a cause of alk depletion, but low CO2/high pH is a prime factor in precipitation of calcium carbonate, which shows up most readily as alk consumption.

Is the sand hardening at all?

Rising nitrate depletes alk, What is the nitrate level?

Many sorts of organisms besides corals also deplete alk, including coralline algae.

A drop of 1.1 dKH per day is not unusual. Even a soft coral tank can use 2 dKH per day if there is decent coralline growth.

Creggers · Mar 23, 2023

Randy Holmes-Farley said:
That change in magnesium is test error or a large salinity change. Magnesium NEVER moves up and down 200 ppm unless it is a big salinity change.

Excess CO2 is never a cause of alk depletion, but low CO2/high pH is a prime factor in precipitation of calcium carbonate, which shows up most readily as alk consumption.

Is the sand hardening at all?

Rising nitrate depletes alk, What is the nitrate level?

Many sorts of organisms besides corals also deplete alk, including coralline algae.

A drop of 1.1 dKH per day is not unusual. Even a soft coral tank can use 2 dKH per day if there is decent coralline growth.

I've never considered nitrate as a contributing factor - I just ordered a test kit off amazon cause that's a mystery to me. I just did a large water change about a week or so ago and have been very diligent about that so I'd be surprised if that's the reason for it.

The sand isn't hard at all. I'm happy to hear that a 1.1dkh fall isn't abnormal but needing to dose as much as I am is sure confusing >.<

Thanks for your reply and I'll look into Nitrate as a potential cause!

Randy Holmes-Farley · Mar 23, 2023

Here's from one of my articles:

Alkalinity Decline in the Nitrogen Cycle

One of the best known chemical cycles in aquaria is the nitrogen cycle. In it, ammonia excreted by fish and other organisms is converted into nitrate. This conversion produces acid, H+ (or uses alkalinity depending on how one chooses to look at it), as shown in equation 1:

NH3 + 2O2 --> NO3- + H+ + H2O

For each ammonia molecule converted into nitrate, one hydrogen ion (H+) is produced. If nitrate is allowed to accumulate to 50 ppm, the addition of this acid will deplete 0.8 meq/L (2.3 dKH) of alkalinity.

However, the news is not all bad. When this nitrate proceeds further along the nitrogen cycle, depleted alkalinity is returned in exactly the amount lost. For example, if the nitrate is allowed to be converted into N2 in a sand bed, one of the products is bicarbonate, as shown in equation 2 (below) for the breakdown of glucose and nitrate under typical anoxic conditions as might happen in a deep sand bed:

(2) 4NO3- + 5/6 C6H12O6 (glucose) + 4H2O --> 2 N2 + 7H2O + 4HCO3- + CO2

In equation 2 we see that exactly one bicarbonate ion is produced for each nitrate ion consumed. Consequently, the alkalinity gain is 0.8 meq/L (2.3 dKH) for every 50 ppm of nitrate consumed.

Likewise, equation 3 (below) shows the uptake of nitrate and CO2 into macroalgae to form typical organic molecules:

(3) 122 CO2 + 122 H2O + 16 NO3- --> C106H260O106N16 + 138 O2 + 16 HCO3-

Again, one bicarbonate ion is produced for each nitrate ion consumed.

It turns out that as long as the nitrate concentration is stable, regardless of its actual value, there is no ongoing net depletion of alkalinity. Of course, alkalinity was depleted to reach that value, but once it stabilizes, there is no continuing alkalinity depletion because the export processes described above are exactly balancing the depletion from nitrification (the conversion of ammonia to nitrate).

There are, however, circumstances where the alkalinity is lost in the conversion of ammonia to nitrate, and is never returned. The most likely scenario to be important in reef aquaria is when nitrate is removed through water changes. In that case, each water change takes out some nitrate, and if the system produces nitrate to get back to some stable level, the alkalinity again becomes depleted.

If, for example, nitrate averages 50 ppm at each water change, then over the course of a year with 10 water changes of 20% each, the alkalinity will be depleted by 1.6 meq/L (4.5 dKH) over the course of that entire time period. This process is one of the primary reasons that fish-only aquaria that often export nitrate in water changes need occasional buffer additions to replace that depleted alkalinity

Creggers · Mar 23, 2023

Randy Holmes-Farley said:
Here's from one of my articles:

Alkalinity Decline in the Nitrogen Cycle

One of the best known chemical cycles in aquaria is the nitrogen cycle. In it, ammonia excreted by fish and other organisms is converted into nitrate. This conversion produces acid, H+ (or uses alkalinity depending on how one chooses to look at it), as shown in equation 1:

NH3 + 2O2 --> NO3- + H+ + H2O

For each ammonia molecule converted into nitrate, one hydrogen ion (H+) is produced. If nitrate is allowed to accumulate to 50 ppm, the addition of this acid will deplete 0.8 meq/L (2.3 dKH) of alkalinity.

However, the news is not all bad. When this nitrate proceeds further along the nitrogen cycle, depleted alkalinity is returned in exactly the amount lost. For example, if the nitrate is allowed to be converted into N2 in a sand bed, one of the products is bicarbonate, as shown in equation 2 (below) for the breakdown of glucose and nitrate under typical anoxic conditions as might happen in a deep sand bed:

(2) 4NO3- + 5/6 C6H12O6 (glucose) + 4H2O --> 2 N2 + 7H2O + 4HCO3- + CO2

In equation 2 we see that exactly one bicarbonate ion is produced for each nitrate ion consumed. Consequently, the alkalinity gain is 0.8 meq/L (2.3 dKH) for every 50 ppm of nitrate consumed.

Likewise, equation 3 (below) shows the uptake of nitrate and CO2 into macroalgae to form typical organic molecules:

(3) 122 CO2 + 122 H2O + 16 NO3- --> C106H260O106N16 + 138 O2 + 16 HCO3-

Again, one bicarbonate ion is produced for each nitrate ion consumed.

It turns out that as long as the nitrate concentration is stable, regardless of its actual value, there is no ongoing net depletion of alkalinity. Of course, alkalinity was depleted to reach that value, but once it stabilizes, there is no continuing alkalinity depletion because the export processes described above are exactly balancing the depletion from nitrification (the conversion of ammonia to nitrate).

There are, however, circumstances where the alkalinity is lost in the conversion of ammonia to nitrate, and is never returned. The most likely scenario to be important in reef aquaria is when nitrate is removed through water changes. In that case, each water change takes out some nitrate, and if the system produces nitrate to get back to some stable level, the alkalinity again becomes depleted.

If, for example, nitrate averages 50 ppm at each water change, then over the course of a year with 10 water changes of 20% each, the alkalinity will be depleted by 1.6 meq/L (4.5 dKH) over the course of that entire time period. This process is one of the primary reasons that fish-only aquaria that often export nitrate in water changes need occasional buffer additions to replace that depleted alkalinity

I never realized the nitrogen cycle impacts alkalinity that way! Thanks for providing that excerpt from your article. I have some test kits coming soon to test all aspects of the cycle cause that's been a blind spot (outside of ammonia) to me. Thanks!!