Set point pH experiment

[chimbo84]

I know this has been discussed a number of times but I have yet another question about pH vs. Alk stability.
A couple months ago I decided to run a little experiment on my tank by using Kalk slurry and dosing to a set pH. I told myself that I would let Alk do it’s thing and if things got too out of hand, I would adjust my AFR dosing to compensate.
My results have so far been quite positive and I am seeing some renewed vitality and coloration in my corals. The only problem is that I cannot seem to prevent my alkalinity from declining and I am wondering if this is the result of increased coral growth or potentially something else.
When I started, my pH would range from 7.7 - 8.1 and now it is pegged to 8.35 (which it maintains pretty well). I started by pegging my pH to 8.1 and have slowly raised that set point over the last couple months in .3 increments. While doing so, I would expect my alkalinity to go up as the higher set point requires more Kalk dosing. However, the opposite has occurred.

Again, my corals seem quite happy despite the low alkalinity level. Is this something that I should try to correct or should this be expected?

 

[Batt]

I know this has been discussed a number of times but I have yet another question about pH vs. Alk stability.
A couple months ago I decided to run a little experiment on my tank by using Kalk slurry and dosing to a set pH. I told myself that I would let Alk do it’s thing and if things got too out of hand, I would adjust my AFR dosing to compensate.
My results have so far been quite positive and I am seeing some renewed vitality and coloration in my corals. The only problem is that I cannot seem to prevent my alkalinity from declining and I am wondering if this is the result of increased coral growth or potentially something else.
When I started, my pH would range from 7.7 - 8.1 and now it is pegged to 8.35 (which it maintains pretty well). I started by pegging my pH to 8.1 and have slowly raised that set point over the last couple months in .3 increments. While doing so, I would expect my alkalinity to go up as the higher set point requires more Kalk dosing. However, the opposite has occurred.

View attachment 3135033
Again, my corals seem quite happy despite the low alkalinity level. Is this something that I should try to correct or should this be expected?

Doing kalk Slurry is dangerous. If you don't have a alk tester that test 4 times a day you can nuke your corals. When doing slurry it drive up alk fast and if you don't have something monitoring it and turning off Surry when you alk goes to high. Then your playing with fire. Luca stated you can't do slurry without having something monitor you alk and controlling it. Don't know if your doing this. Hope you are. Luca also recommend using the alkatronics to test alk Frequently

 

[chimbo84]

Doing kalk Slurry is dangerous. If you don't have a alk tester that test 4 times a day you can nuke your corals. When doing slurry it drive up alk fast and if you don't have something monitoring it and turning off Surry when you alk goes to high. Then your playing with fire. Luca stated you can't do slurry without having something monitor you alk and controlling it. Don't know if your doing this. Hope you are. Luca also recommend using the alkatronics to test alk Frequently

This isn’t an issue for me. I have the Neptune trident testing four times a day and constant pH monitoring. That’s what the graph is from in my original post.

 

[Stephen Glover]

Probably a bit of precipitation and coral growth

 

[Randy Holmes-Farley]

There are two feedback loops that help prevent alk from rising too fast as more hydroxide is dosed, and both are caused by pH increasing.

Abiotic precipitation of calcium carbonate rises as pH rises, as does calcification by some corals.

Thus, there are scenarios where dosing more leads to lower alkalinity. Not always, but sometimes.

An extreme example if a big limewater/kalkwasser overdose, where the whole tank can turn white from preciptitated calcium carbonate, and the resulting alk can often be lower than before the precipitation event

 

[chimbo84]

There are two feedback loops that help prevent alk from rising too fast as more hydroxide is dosed, and both are caused by pH increasing.

Abiotic precipitation of calcium carbonate rises as pH rises, as does calcification by some corals.

Thus, there are scenarios where dosing more leads to lower alkalinity. Not always, but sometimes.

An extreme example if a big limewater/kalkwasser overdose, where the whole tank can turn white from preciptitated calcium carbonate, and the resulting alk can often be lower than before the precipitation event

Thanks for the response Randy! My water just tested at 6.4dKh alkalinity and a pH of 8.35. I haven't noticed an increase in precipitation but this low alkalinity is starting to make me nervous. What I am going to do is slowly roll back my pH in .05 increments and see if I can get my alkalinity back over 7.5dKh and settle there.

I did notice when I first started this experiment, my Alkalinity shot up to over 9 and as I held the pH, it slowly came down over the course of a week or so. Was this the result of the additional kalkwasser driving CO2 out of the tank? I am wondering, could I increase alkalinity at a given pH by increasing the amount of dissolved CO2?

 

[Randy Holmes-Farley]

I did notice when I first started this experiment, my Alkalinity shot up to over 9 and as I held the pH, it slowly came down over the course of a week or so. Was this the result of the additional kalkwasser driving CO2 out of the tank? I am wondering, could I increase alkalinity at a given pH by increasing the amount of dissolved CO2?

That drop was probably from increased consumption by organisms and by abiotic precipitation.

There's a bit of a chicken and the egg issue here. CO2, pH and alk are all related mathematically and you cannot move all three independently.

Adding CO2 lowers pH at fixed alk. Adding alk at a fixed pH will raise alk and CO2.

If you want alk higher, I'd add more of a lower pH alk additive along with what you are doing now. That may or may not work, since abiotic precipitation may just ramp up, and you may be forced to back down on your pH goals.