alkalinity drops after baking soda dosing

[SnowD]

These days, I have a hard time to make alkalinity stable in my 6 g reef tank with 4 g sump. This is how the problem started.

I do 2 g water change every 3 weeks. I use kalk in ATO for Alk and Ca. My parameter was fairly stable for long time (1.5 yrs old tank).

Alk: 7 (API)
Ca: >500 (Selifert) [I didn't correct it because it seems OK when I searched online]
Mg: 1350 (Selifert)
PH: 8.4 (API)
NO3: ~2-5 ppm (API)
PO4: ~0 (API)

A month ago, my ATO pump wasn't working for half a day, and the return pump also stopped for some reason. After fixing everything I measured parameters again. All were OK; but my alk was about 6 dkH (dropped from 7).

I thought it's a good time to raise my alk to 8-9 range because I have 2-5 ppm NO3 and wanted to accelerate coral growth a little bit. Coraline algae never grow in my tank, so, I thought raised alk might solve the problem too.


So I supplemented alk using baking soda. I used Reef Calculator to measure the amount of dose. http://reef.diesyst.com/chemcalc/chemcalc.html

For three days, I dosed extra alkalinity of about 1 dkH/day (1.1 g of baking soda) . My alk was always at 7 dkH before, so I expected it to be about 9 dkH (from 6 to 9). But it kept dropping ( Ca level dropped very quickly too). Despite of baking soda dosing, alk stayed in about 6.5-7 dkH range. I am puzzled. Why does the alk not increase even though I extra dose?

I believe this means my tank now needs extra alk more than 1 dkH/day on top of kalk. Is this normal? I suspect that when I raised alk, it triggered some extra growth of something like algae or snails in my tank, and then it boost alk consumption too. Is the right explanation? I just don't understand.

Now I can't make alk stable anymore with kalk. Is adding two-part dosing system the only solution here? Or is there any other way to try?

I'd like to keep alk about 9 dkH. I really appreciate your advice.

 

[CastAway]

Tagging along.

In a system the size of yours, I would think water changes alone would be best to keep your levels where they ought to be.

I would imagine dosing small volumes would be tough.

Any white residue on the glass?

 

[SnowD]

Thanks CastAway. I could do water change more frequently. But I just want to understand why alk doesn't increase in my tank even with extra dosing. Before the extra dosing, there was no problem with kalk in ATO.

Alk stayed 7 dkH with kalk. So assuming alk consumption is the same, extra dose of 2 dkH will bring alk to 9 dkH, and it will stay there with kalk, right?

 

[Larry L]

For three days, I dosed extra alkalinity of about 1 dkH/day (1.1 g of baking soda) . My alk was always at 7 dkH before, so I expected it to be about 9 dkH (from 6 to 9). But it kept dropping ( Ca level dropped very quickly too).

Since your calcium also dropped quickly, I bet when you added the baking soda it was combining with the calcium and precipitating out of the water as calcium carbonate. How did you add the baking soda? Normally should dilute it in R/O water, and then add it very slowly in an area with lots of flow. Also, might want to consider bringing it up more slowly than 1 dKH per day - a lot of people recommend raising by no more than 0.5 dKH per day.

 

[SnowD]

Since your calcium also dropped quickly, I bet when you added the baking soda it was combining with the calcium and precipitating out of the water as calcium carbonate. How did you add the baking soda? Normally should dilute it in R/O water, and then add it very slowly in an area with lots of flow. Also, might want to consider bringing it up more slowly than 1 dKH per day - a lot of people recommend raising by no more than 0.5 dKH per day.

I dilute baking soda to RO/DI water and then added it around drain in sump. I tried to raise 1 dkh in a day. Divide the solution three parts and added it with two hours interval. For example, the starting Alk was 7 dkh in the morning, and it increases to 7.5 or 8 dkH during the day. But when I measure Alk again in the next morning, it becomes 6.5 or 7 dkh. So I don't actually raise full 1 dkh/day. It can't barely stay in the same level of Alk with the baking soda dose on top of Kalk.

However, the precipitating could be the reason if there shouldn't be such big changes in alk consumption.

Now I'm trying to use dripper. Slow dripping might solve the the rapid calcuim carbonate precipitation issue. So far, I am still not able to keep Alk steady without Alk supplement.

 

[Larry L]

I've also heard that low magnesium can contribute to precipitation, but if you are still around 1350 that should not be an issue.

 

[Randy Holmes-Farley Verified Community Expert]

Demand rises rapidly as the alkalinity rises. So at 9 dKH you expect a faster drop than at 7 dKH.

I suspect that is all the issue is, but if you initiated some precipitation somehow, then the fresh crystals can act as nucleating sites for further precipitation, and a cycle begins that can be hard to stop.

 

[SnowD]

Demand rises rapidly as the alkalinity rises. So at 9 dKH you expect a faster drop than at 7 dKH.

I suspect that is all the issue is, but if you initiated some precipitation somehow, then the fresh crystals can act as nucleating sites for further precipitation, and a cycle begins that can be hard to stop.

Thanks Randy. That makes sense.
So considering the issue in my tank, you don't think keeping Alk at 9dKH in my tank is not possible with only kalk? Another question is, if the precipitation was initiated, how can I remove the crystals or prevent further precipitation?

 

[Randy Holmes-Farley Verified Community Expert]

Thanks Randy. That makes sense.
So considering the issue in my tank, you don't think keeping Alk at 9dKH in my tank is not possible with only kalk? Another question is, if the precipitation was initiated, how can I remove the crystals or prevent further precipitation?

Adding 1 dKH per day isn't that much (even on top of the limewater) .

First thing I'd do if you want to get to 9 dKH is dose 0.75 dKH twice a day and see what that accomplishes after a few days (or spread out even more if you want, such as dosing pump or dripper).

 

[SnowD]

I set a dripper a couple of days ago and get relatively stable Alk. It's about 8 dKH now.

Since I added Alk supplement diluted in RO/DI, Kalk in ATO is also slightly less added. I did math. It turns out that Alk supply from Kalk is 1.35 dKH/day and now I'm adding about 1.5 dKH/day by Alk supplement. So my tank consumes 2.85 dKH/day now to sustain 8 dKH.

In conclusion, to raise Alk 7 dKH to 8 dKH, it's not like simply adding 1 dKH one time. While one time 1 dKH bump will bring 7 to 8 dKH, it will naturally decrease to 7 dKH again or even lower than 7 dKH in a day because of some CaCO3 precipitation or elevated growth. To sustain it at 8 dKH, I need to add 1.35 dKH/day more Alk supplement. Basically if you are using Kalk with full strength, you can't raise Alk without constant Alk supplement by extra.

Detailed numbers will be only valid for my tank. But I hope this information is useful to others who want to raise Alk.

 

[PAXpress]

I had tons of trouble with my 75g with 21g sump when I was using Kalk. I would manually dose etc. I couldn't get alk to stay above 6 dkh. Only thing that fixed it was not using kalk and switching to baking soda instead of soda ash. Haven't had a problem since. Good luck!

 

[CastAway]

Demand rises rapidly as the alkalinity rises.

I wish I understood this. Is the demand from calcifying corals only?

My CA reactor doesn’t seem to keep up anymore, and I try to supplement with a dissolved baking soda drip on occasion. Indeed the effect is very short lived.

 

[Randy Holmes-Farley Verified Community Expert]

I wish I understood this. Is the demand from calcifying corals only?

My CA reactor doesn’t seem to keep up anymore, and I try to supplement with a dissolved baking soda drip on occasion. Indeed the effect is very short lived.

Both abiotic precipitation of calcium carbonate and coral calcification rates rise as pH rises and also as alkalinity rises.

All additives at least have the alk effect, and many have the pH effect, especially limewater which has the max possible pH rise per unit of alkalinity added..