Solving an Alkalinity swing problem in a nano tank

[natickwaterbox]

I’m trying to understand, and potentially fix, an observed alkalinity swing in a 10 gallon SPS nano tank that occurs over a 48 hour time period. The total volume of the tank is likely closer to 8 gallons. It is very heavily stocked with SPS and has a substantial amount of plating coralline algae.

What I observe is that my alkalinity is 8.3 dKH at 5pm on Day 1, on Day 2 my alkalinity is 6.5 dKH at 5pm, and finally on Day 3 at 5pm the value returns to 8.3 dKH. The pattern repeats itself ad infinitum. I am replacing Alkalinity and Calcium using BRS 2-part solution (soda ash and calcium chloride). I am using standard BRS dosing pumps on reef keeper timer that should dose roughly 0.5ml of each solution every 30 minutes for a total volume of each solution of roughly ~24ml/day. I notice that my pH is a very good indicator of my alkalinity in my system – my pH swings from 8.55 to 8.35. When my pH is 8.55 my alkalinity is 8.3 dKH and when the pH is 8.35 my alkalinity is generally around ~6.5 dKh.

My system receives 3.5% daily automatic water changes with standard IO.

Other measurements that might be helpful:

Most recent measurements:
Salinity 1.026 sg
Temp - 78F
Mg - 1380 ppm
Ca – 385 ppm (low I know, my Ca has a tendency to trend down…)
Nitrate – 3 ppm
PO4 – 0.03ppm

Hannah checkers for Alk/PO4, RedSea for Mg/Ca, NyOS for Nitrate

Things I have tried:

(1) Completely stopped dosing and slowly ramping up again. Same 48 hour swing returns.

(2) Completely changed dosing solutions and dosing lines. No effect.

I have not lost any corals over the 4 months since I have been closely monitoring this dKh swing. Actually, everything looks pretty good! But I’m sure some corals could be growing faster.

I’m sure it’s not ideal to have such a swing over a 48 hour period, however, given the small water volume and the abundance of Coralline/SPS it might be something one has to live with in such a tank? Hoping you guys might be able to provide some input. Thanks.

 

[Randy Holmes-Farley Verified Community Expert]

I don't see a reason for the swing you see if you are dosing evenly. There can be less demand at night, so dosing more during the day can stabilize things, but I see no reason for a 48 h cycle.

Since the pH runs fairly high (if accurate), switching to bicarbonate instead of carbonate (or aerating more) will reduce the pH and potentially reduce the amount of abiotic precipitation of calcium carbonate.

 

[natickwaterbox]

Yes, the 48 hour cycle is bizarre.

I don't know how much I trust the BRS dosing pumps to deliver precise equal volumes of solutions. Can small differences in dosing volumes produce this type of 48 hour cycle?

I don't see much abiotic precipitation. I'm sure it occurs but I don't see obvious signs on heaters/pumps.

What are your thoughts on the Calcium trending downward? If my Ca dosing pump was delivering 5-10% less solution compared to the Alkalinity pump could you get a 48 hour cycle like this?

 

[LobsterOfJustice]

My only thought is...

Day 1, alk is high and corals are happy so they calcify. This drops the alkalinity despite your dosing (they use more than you replace).

Day 2, alk is low, corals are not happy and do not calcify. This raises alkalinity because you continue dosing but corals are not calcifying.

Repeat...

 

[natickwaterbox]

Lobster: That was my interpretation too for awhile. But that rationale implies, I think, that I am not supplying enough 2-part to meet the demands of the tank. I tried increasing 2 part 20% daily but that did not mitigate the swing (perhaps just increased abiotic precipitation?).

Thats when I began suspecting that I might be supplying unequal 2 part solutions due to either: 1) Me not mixing the chemicals correctly or 2) line blockage or 3) differences in pump efficiency to deliver solutions.

I suppose a simple fix would be to measure the total volumes the pumps can deliver if allowed to run for 30 minutes into a cup.... That should allow me to quantitatively measure efficiency.

 

[Randy Holmes-Farley Verified Community Expert]

Yes, the 48 hour cycle is bizarre.

I don't know how much I trust the BRS dosing pumps to deliver precise equal volumes of solutions. Can small differences in dosing volumes produce this type of 48 hour cycle?

That would seem to cause hourly variations, not 48 h variations.

What are your thoughts on the Calcium trending downward? If my Ca dosing pump was delivering 5-10% less solution compared to the Alkalinity pump could you get a 48 hour cycle like this?

Dosing pumps are not always exact, so if you go by timer and not volume, they can easily be off.

 

[mav3rick478]

Have you tried not doing the daily water changes for a week and seeing if your ALK still fluctuates?

 

[natickwaterbox]

I started the automatic daily water changes after noticing this 48 hour cycle. It doesn't seem to effect it at all.

So, my plan of action based on the feedback is as follows:
1) Run a 30 minute test on each dosing pump to validate the efficiency of each dosing pump to dispense an equal volume of solution over the total time of daily dosing.
2) Modify the dosing times to coincide with the time of highest Alk/Ca demand (ie day).
3) Recalibrate my pH probe to verify the pH values are accurate.

I purchased a GHL 2.1 doser for another tank. I'm thinking I might set this up on the 10gallon and monitor Alk for a similar swing to rule out the BRS dosing pump as the problem.

One final question, could "over dosing" 2 part lead to this cycle? It is technically possible that I am dosing too much 2-part.

 

[Randy Holmes-Farley Verified Community Expert]

I cannot think of anything that can lead to a 48 h cycle unless dosing is not as uniform as you think, or something else is varying, such as the CO2 in the room, etc.

 

[Flatlandreefer]

Tagging along, I too have trouble maintaining rock solid alk numbers in my 25 gallon nano. Seems like consumption will increase for a few days and then all of a sudden slows down and my dkh creeps up.

 

[natickwaterbox]

Update:
So, I calculate that each pump runs for roughly 27.2 minutes over a 24 hour time period. To test the efficiency of each pump I ran each pump into a clean plastic cup, for 27 minutes, then stopped the pump and measured the volume of each solution using a serological pipette.
Alk pump yielded a volume of 37.8 ml. (Significantly higher than the 24ml I quoted above)
Calcium pump yielded a volume of 37.9 ml.

Well, there goes the crappy pump hypothesis... I'm seeing a dosing rate of 1.4 ml/min - significantly higher than the 1.1 ml /minute reported by BRS.

Tonight, I'll switch the dosing times to coincide with the light cycle (12 hour period). So, 24 doses of 68 seconds every 30 minutes.

 

[natickwaterbox]

I've attached some raw data I've inputted into Aquarimate for Alk and pH. Sometimes life gets in the way of me taking measurements everyday but you can see the trend I spoke of in the first post. Pay attention to the measurements in early March as those were taken at the same time every day for over a week...

 

[mav3rick478]

Is the dosing necessary with such a small tank and the daily water changes? I'm just asking because I don't really keep SPS, more of an LPS guy.

 

[natickwaterbox]

Yes, there is massive Alkalinity consumption - I think it is largely due to the plating coralline algae. I'll post a picture of the tank at some point...

 

[natickwaterbox]

Randy in regards to the CO2 possibility.

The tank is located in a 2000 square foot home. There are two people living in the home and we are only home from 5pm - 7am. The tank is located near an outside door that is opened frequently. 1 small 5lb dog home all the time...

 

[Randy Holmes-Farley Verified Community Expert]

Randy in regards to the CO2 possibility.

The tank is located in a 2000 square foot home. There are two people living in the home and we are only home from 5pm - 7am. The tank is located near an outside door that is opened frequently. 1 small 5lb dog home all the time...

Not home more on some days like weekends? Might the weather patterns have been such that for a short period, CO2 might have varied periodically?

 

[natickwaterbox]

Yes, we are home more on weekends. I have not noticed changes in Alk/pH on weekends versus during the weekdays. Although I will pay more attention to that in the future.

 

[reeferfoxx]

@natickwaterbox How do you have the reefkeeper setup? Do you program it? Is it a simple timer? I'm not familiar with it but I would make sure it's set for the daily and not every other day. Also, any other factors that might effect power or something in the house while you're away?

As long as the dosers are consistently and equally dosing each part, there shouldn't be a drastic swing.

 

[LobsterOfJustice]

Lobster: That was my interpretation too for awhile. But that rationale implies, I think, that I am not supplying enough 2-part to meet the demands of the tank. I tried increasing 2 part 20% daily but that did not mitigate the swing (perhaps just increased abiotic precipitation?).

The fact that your alk drops from 8.3 to 6.5 means on some level, you’re not keeping up with demand. It sounds like overall you might be adding an appropriate amount, but you could shift around the timing of the additions. I would try to spend a few days and test at multiple points throughout the day. You might need to back off dosing at some times that your demand is lower and make up the difference when you see a drop due to higher demand.

 

[natickwaterbox]

Ha! That is a clever explanation! Admittedly, I'm not a Reefkeeper expert when it comes to the programming...

So, each dosing pump is on its own outlet and each is set to a simple timer:
Day of the Week:SMTWTFS
Start time: 12:01am
duration: 34 seconds
Time off: 29:30
Repeat: 48
Oscillate: off
Random: No.

The other pump On time is 12:05am, same program.

I think it's okay. Although I need to check exactly what the oscillate and random settings do.