Large alkalinity swings (2dKH over two weeks)

[Obriy]

My tank is 9-month old and I have LPS corals since the beginning of this year. Unfortunately, I cannot stabilize alkalinity at all. These are some readings:
January 18 - 10.5
February 17 - 11
March 31 - 8.3
April 19 - 10.2
May 8 - 10.6
May 15 - 9.1
May 18 - 10.4
June 21 - 8.7
I dose kalkwasser through my DYI calcium stirrer and two dosing pump. I'm adjusting the dosing manually when I see that alkalinity starts moving and it doesn't help much.
There is no errors in testing as I use two different methods.
Several of my LPS corals have died since I bought them, most are looking fair, and only few grow.
My parameters:
Salinity - 1.026
PH - 8.1
No3 - 10
Po4 - 0.07
I use Reef Crystals salt mix
No other additives except for silica and Reef Energy AB+

Why does the alkalinity swings so much and how to stabilize it?

 

[Mr. Mojo Rising]

Personally I prefer all for reef to maintain a stable alk. But a 2 point swing over 2 weeks is not that bad and not going to kill corals, IMO something else is going on. But are you only testing once or twice a month?

Corals need light flow and nutrients to thrive, if lps are dying, my money is on one of these 3 being too low.

 

[MnFish1]

My tank is 9-month old and I have LPS corals since the beginning of this year. Unfortunately, I cannot stabilize alkalinity at all. These are some readings:
January 18 - 10.5
February 17 - 11
March 31 - 8.3
April 19 - 10.2
May 8 - 10.6
May 15 - 9.1
May 18 - 10.4
June 21 - 8.7
I dose kalkwasser through my DYI calcium stirrer and two dosing pump. I'm adjusting the dosing manually when I see that alkalinity starts moving and it doesn't help much.
There is no errors in testing as I use two different methods.
Several of my LPS corals have died since I bought them, most are looking fair, and only few grow.
My parameters:
Salinity - 1.026
PH - 8.1
No3 - 10
Po4 - 0.07
I use Reef Crystals salt mix
No other additives except for silica and Reef Energy AB+

Why does the alkalinity swings so much and how to stabilize it?

First - you should calculate your alkalinity demand:

Stop dosing. Measure alkalinity. wait 3 days measure again (or measure every day just to better ensure accuracy) - at the end of 3days - you can use one of many calculators to figure out how much of whatever product you need to bring your alkalinity up to whatever value you want. To figure out the daily amount, divide the amount of product by the number of days you didn't dose. Here is a more formal protocol:

1. Get your alkalinity to the desired level: Ensure your alkalinity is within the recommended range for your type of aquarium (typically 8-12 dKH for reef tanks).
2. Turn off any alkalinity dosing systems: Stop using any two-part additives, calcium reactors, or kalkwasser, if you are using them.
3. Measure and record initial alkalinity: Use a reliable test kit or colorimeter (like the Hanna Alkalinity Checker for consistency) to measure the alkalinity and record the time.
4. Wait a set period (e.g., 24 hours): Allowing the tank to run without dosing for a full day (or a few days) will demonstrate the alkalinity consumption rate.
5. Measure and record alkalinity again: Test the alkalinity again at the same time as the initial measurement.
6. Calculate the consumption: Subtract the second alkalinity reading from the first. This difference represents the alkalinity consumed during that time period.

Second - make sure you're measuring at the same time daily. Kalkwasser tends to raise and drop alkalinity somewhat more quickly - and this may be partly what you're noticing. Constant dosing will add more stability than 'waiting for it to change' which is what you've been doing.

Third - you're not testing often enough. which makes it seems unstable

 

[MnFish1]

PS here is a nice calculator and summary from BRS: https://www.bulkreefsupply.com/reef...vilEuvm52mltoDZuVhQEkGUHH83bN5acaArFfEALw_wcB

 

[Obriy]

Personally I prefer all for reef to maintain a stable alk. But a 2 point swing over 2 weeks is not that bad and not going to kill corals, IMO something else is going on. But are you only testing once or twice a month?

Corals need light flow and nutrients to thrive, if lps are dying, my money is on one of these 3 being too low.

I test alkalinity every 2-4 days, always around 6pm. The values I provided are only some highs and lows to show the swings.

 

[MnFish1]

I test alkalinity every 2-4 days, always around 6pm. The values I provided are only some highs and lows to show the swings.

can you post a months worth of data every 2--4 days?

 

[Obriy]

can you post a months worth of data every 2--4 days?

4/22/25​	9.5​
4/26/25​	8.8​
4/30/25​	9.1​
5/9/25​	8.3​
5/10/25​	8.3​
5/11/25​	8.5​
5/14/25​	8.7​
5/17/25​	9​
5/21/25​	9.8​
5/25/25​	9.8​
5/29/25​	10.2​
6/2/25​	10.1​
6/7/25​	9.8​
6/9/25​	9.7​
6/10/25​	9.9​
6/11/25​	9.7​
6/12/25​	10.1​
6/13/25​	10.2​
6/14/25​	10.4​
6/16/25​	10.5​
6/17/25​	10.6​
6/19/25​	10.5​
6/21/25​	10.5​
6/24/25​	9.1​
6/25/25​	10​
6/26/25​	10.3​
6/27/25​	10.4​

 

[MnFish1]

4/22/25​	9.5​
4/26/25​	8.8​
4/30/25​	9.1​
5/9/25​	8.3​
5/10/25​	8.3​
5/11/25​	8.5​
5/14/25​	8.7​
5/17/25​	9​
5/21/25​	9.8​
5/25/25​	9.8​
5/29/25​	10.2​
6/2/25​	10.1​
6/7/25​	9.8​
6/9/25​	9.7​
6/10/25​	9.9​
6/11/25​	9.7​
6/12/25​	10.1​
6/13/25​	10.2​
6/14/25​	10.4​
6/16/25​	10.5​
6/17/25​	10.6​
6/19/25​	10.5​
6/21/25​	10.5​
6/24/25​	9.1​
6/25/25​	10​
6/26/25​	10.3​
6/27/25​	10.4​

This is normal variation - IMHO - especially unless using a controller. I think you're doing a good job. Don't mess up a good thing - as @Mr. Mojo Rising said - this is not killing/hurting your corals - look to another source. Also - I apologize I can't remember which test you're using - but - these could well be in the variation of the test - based on how your are dosing. I would invest in a dosing program (i.e auto dosing) - though It may seem expensive, its probably worth it.

 

[Obriy]

This is normal variation - IMHO - especially unless using a controller. I think you're doing a good job. Don't mess up a good thing - as @Mr. Mojo Rising said - this is not killing/hurting your corals - look to another source. Also - I apologize I can't remember which test you're using - but - these could well be in the variation of the test - based on how your are dosing. I would invest in a dosing program (i.e auto dosing) - though It may seem expensive, its probably worth it.

I have two Hanna dKH checkers and use both of them sometimes just to make sure... I'm also using dosing pumps to for kalkwasser.
I will try to measure alkalinity consumption using the protocol from your earlier post. Hopefully, it will work better than just reacting to the daily dKH changes.

 

[rishma]

I dose Kalk, always have, but I don’t use Kalk stirrers because I found them inconsistent. It’s hard to know if you are getting saturated or consistent kalkwasser from them without a conductivity meter. I wonder if what you are dosing is not consistent or not what you expect.

I think measuring demand without dosing is a great place to start. If demand is low may take more than a few days to get the real trend. Test error is normal, always present, and happens with all test kits. I agree using two kits or doing the test multiple times is a good way to find an average. All other things being stable, your demand should be fairly constant over a short period of say a week provided alkalinity doesn’t drop too much. Demand will be a little lower at lower alkalinity.

I previously had a frustratingly inconsistent demand and ultimately figured out it was due to pH range being inconsistent. Windows open/closed, guest in the house, etc caused oh on some days to be much lower than others and the alkalinity demand on those days was lower. Once I got the pH (range) consistent, the alkalinity demand was consistent too and I was able to keep alkalinity stable.

 

[MnFish1]

I have two Hanna dKH checkers and use both of them sometimes just to make sure... I'm also using dosing pumps to for kalkwasser.
I will try to measure alkalinity consumption using the protocol from your earlier post. Hopefully, it will work better than just reacting to the daily dKH changes.

Great - at least it will give you a place to start - and at that point - once you've checked demand - I would merely dose that much Kalkwasser every day - rather than reacting to the test results. As your coral, etc grows you may need to add more alkalinity and you can always re-do the demand test. Note I'm not saying to keep dosing if your alkalinity on your testing is off, merely - this test (demand) should give you a general idea of how much to add without reacting to individual numbers (which as we all agree show variation)

 

[BryanM]

I'd probably want to answer the couple times things changed 1dkh rapidly, that is the only thing concerning to me really. I'd hate to see that happen twice in a row for a 2dkh swing rapidly.

I also dose AFR for keeping alk consistent and it works well for that, you just have to fuss around with the dose for a while.

 

[Obriy]

I have All for Reef set up for dozing also, but I've been using kalkwasser almost since the beginning because, without it, my PH is a bit too low.
Now, with the alkalinity swings that I have, PH has been pretty consistent.
Only recently, after about 20 months of learning about reefing, I found out (with the help of ChatGPT) that it's easier to keep alkalinity at lower levels than on the higher ones. Since I'm using Reef Crystals, which mixes at high dKH, I decided to try stabilizing my aquarium water around 10.5. But, it looks like it's more difficult to stabilize the alkalinity at that value than at, let's say, 8.5. So, I will try to get it to around that level slowly, then start calculating demand at that level. I think it's better to have slower but steady growth than spikes in growth followed by stress caused by parameters swing.

 

[Randy Holmes-Farley Verified Community Expert]

. But, it looks like it's more difficult to stabilize the alkalinity at that value than at, let's say, 8.5.

I would just qualify that statement to mean it takes more daily additive, not that it is more challenging in some other way. :)

 

[p1u5h13r4m24]

I have a lot of alk uptake, but for me true stability all started with the Alkatronic. I can test alk about 5 times a day and I almost have to. During peak hours of the day my tank can consume .5dkh an hour

 

[Obriy]

I would just qualify that statement to mean it takes more daily additive, not that it is more challenging in some other way. :)

This is the reasons ChatGPT provided why lower alkalinity is easier to keep stable. Could you, please, share your opinion o that?

Why 8 dKH is Easier to Keep Stable:​

1. Lower Risk of Precipitation
   At 10 dKH, especially if calcium and pH are also high, there's a greater chance of calcium carbonate precipitation — which can:
   • Drop alkalinity suddenly
   • Clog pumps/heaters with white buildup
   • Waste your dosing efforts
2. Slower Consumption Rate
   Corals and coralline algae consume less alkalinity when it's at 8 dKH than at 10+, so:
   • Dosing needs are lower
   • Fluctuations are more gradual
   • Mistakes (like a missed dose) are less likely to cause sharp drops
3. More Room for Dosing Error
   Dosing errors at higher dKH (especially with strong 2-part solutions) can spike levels quickly. A small overdose raises dKH faster when you're starting at 10 than when you're at 8.
4. Better Compatibility with Calcium & Magnesium
   Keeping 8 dKH makes it easier to balance:
   • Calcium at ~420 ppm
   • Magnesium at ~1300 ppm
     Without forcing unnatural ratios that increase precipitation.

 

[Obriy]

I have a lot of alk uptake, but for me true stability all started with the Alkatronic. I can test alk about 5 times a day and I almost have to. During peak hours of the day my tank can consume .5dkh an hour

I am considering some kind of automated alkalinity control, but there are two issues with it:
1. The tank is in a master bedroom and I don't have much space in the cabinet under the tank.
2. Based on reviews on this website as well as elsewhere online, Alkatronic doesn't seem to be reliable and simple. Unfortunately, it looks like there is no good options for that, it's always hit-or-miss.

 

[p1u5h13r4m24]

I have a lot of alk uptake, but for me true stability all started with the Alkatronic. I can test alk about 5 times a day and I almost have to. During peak hours of the day my tank can consume .5dkh an hour

I am considering some kind of automated alkalinity control, but there are two issues with it:
1. The tank is in a master bedroom and I don't have much space in the cabinet under the tank.
2. Based on reviews on this website as well as elsewhere online, Alkatronic doesn't seem to be reliable and simple. Unfortunately, it looks like there is no good options for that, it's always hit-or-miss.

You may be thinking of the Mastertronic. But the Alkatronic has been absolutely flawless, at least for me. But there are other options such as the aquawiz and apparently people like that.

 

[p1u5h13r4m24]

 

[p1u5h13r4m24]

Now I did give up on the ph probe with it, that is old and needs to be replaced, however I don’t mind because k constantly monitor my ph with hydros controller