Po4 and Kh effects

[scotty333]

Hi guys , for the last 2 months or I’ve been closely monitoring my po4 and do a kh at the same time
I have absolute concrete evidence that when po4 is low my kh drops too , over .1 and it rises the kh

I know this isn’t a newsflash but I wanted others opinions or their experiences too as this has been recognised for a long time but I never really hear anyone talking about the same pattern

@The boss , is there much too back this up ?documented ?

 

[EnterName]

Phosphate makes up a very very small fraction of total alkalinity:

Total Alkalinity = [HCO₃⁻] + 2[CO₃²⁻] + [B(OH)₄⁻] + [OH⁻] + [Si(OH)₃O⁻] + [MgOH⁺] + [HPO₄²⁻] + 2[PO₄³⁻] + [minor species] - [H⁺]​

I'm lazy and Randy already did the calculations: A drop of 1ppm PO₄ will decrease total alkalinity by only 0.03dKH. So it wouldn't be noticeable if you are talking about a 0.1ppm PO₄ drop.

Maybe phosphate precipitation plays a more important role here? I'm not sure.

 

[scotty333]

Phosphate makes up a very very small fraction of total alkalinity:

Total Alkalinity = [HCO₃⁻] + 2[CO₃²⁻] + [B(OH)₄⁻] + [OH⁻] + [Si(OH)₃O⁻] + [MgOH⁺] + [HPO₄²⁻] + 2[PO₄³⁻] + [minor species] - [H⁺]​

I'm lazy and Randy already did the calculations: A drop of 1ppm PO₄ will decrease total alkalinity by only 0.03dKH. So it wouldn't be noticeable if you are talking about a 0.1ppm PO₄ drop.

Maybe phosphate precipitation plays a more important role here? I'm not sure.

Thanks but I’m talking bigger numbers here
I.e my kh sits steady at 9 when po4 is between .05-.09 and it goes up to 9.5 if it creeps above .1
And as low as .02-.05 it goes down to 8.5
I have fluctuating po4 where sometimes I need a quantity of food to maintain the level I want and other times where I need a steady dose of lanthanum to stop them flying up
It’s a pain tbh but I have to keep on top of it

 

[EnterName]

Thanks but I’m talking bigger numbers here
I.e my kh sits steady at 9 when po4 is between .05-.09 and it goes up to 9.5 if it creeps above .1
And as low as .02-.05 it goes down to 8.5
I have fluctuating po4 where sometimes I need a quantity of food to maintain the level I want and other times where I need a steady dose of lanthanum to stop them flying up
It’s a pain tbh but I have to keep on top of it

That's why I assume it might be related to phosphate precipitation (I'm not sure though).

Maybe there is some other reason...

• what are you dosing to maintain calcium and alkalinity?
• what tests are you using to measure phosphate and alkalinity?
• do you use any adsorber materials
• are running a calcium reactor?
• are you dosing nitrates/phosphates sometimes?
• have you been observing this relationship between phosphate and alkalinity with different test kits or at least different/new reagents?

 

[scotty333]

That's why I assume it might be related to phosphate precipitation (I'm not sure though).

Maybe there is some other reason...

• what are you dosing to maintain calcium and alkalinity?
• what tests are you using to measure phosphate and alkalinity?
• do you use any adsorber materials
• are running a calcium reactor?
• are you dosing nitrates/phosphates sometimes?
• have you been observing this relationship between phosphate and alkalinity with different test kits or at least different/new reagents?

I think it’s the rock and sand binding and unbinding but I’ll answer the list and see what shakes out the tree


I use afr plus trace a and k for total dosing
Po4 is Hanna ulr , different batch numbers never vary the results
Salifert for kh , I find it the easiest of Kh testing and always consistent
No gfo or absorbers
No reactor
I have to dose nitrates and is set at 8-10ppm
I always get the same pattern regardless of batches , they could read a small variation when replaced but the trend remains just a slight different number

 

[Randy Holmes-Farley Verified Community Expert]

Thanks but I’m talking bigger numbers here
I.e my kh sits steady at 9 when po4 is between .05-.09 and it goes up to 9.5 if it creeps above .1
And as low as .02-.05 it goes down to 8.5
I have fluctuating po4 where sometimes I need a quantity of food to maintain the level I want and other times where I need a steady dose of lanthanum to stop them flying up
It’s a pain tbh but I have to keep on top of it

I expect there is no cause and effect, but rather are either both responding to the same third causative agent, or it is coincidence.

 

[EnterName]

I think it’s the rock and sand binding and unbinding but I’ll answer the list and see what shakes out the tree


I use afr plus trace a and k for total dosing
Po4 is Hanna ulr , different batch numbers never vary the results
Salifert for kh , I find it the easiest of Kh testing and always consistent
No gfo or absorbers
No reactor
I have to dose nitrates and is set at 8-10ppm
I always get the same pattern regardless of batches , they could read a small variation when replaced but the trend remains just a slight different number

I don't know if this will explain a 0.5dKH difference, but maybe it contributes to the observed phenomenon:

AFR makes use of calcium formate to provide calcium and alkalinity. The formate ion is converted to bicarbonate by bacteria and only then it adds to alkalinity. Maybe increased phosphate levels speed up this process in your tank, so you get the full AFR alkalinity effect faster.

Additionally dosing nitrate directly (e.g. sodium nitrate) instead of dosing ammonia will also slightly increase alkalinity. 50ppm nitrate correspond to approx. 2.3dKH according to the linked article, so 11ppm nitrate would be enough to increase alkalinity by 0.5dKH once assimilated (this effect is delayed because it relies on the nitrate being assimilated by bacteria, algae, corals, etc.).

It's probably a whole set of different factors and I'm sure the suggested options are not enough to explain what is happening. Randy knows this stuff far better than I do, so if he suspects some third causative agent or simply coincidence, he is probably right.

 

[scotty333]

I don't know if this will explain a 0.5dKH difference, but maybe it contributes to the observed phenomenon:

AFR makes use of calcium formate to provide calcium and alkalinity. The formate ion is converted to bicarbonate by bacteria and only then it adds to alkalinity. Maybe increased phosphate levels speed up this process in your tank, so you get the full AFR alkalinity effect faster.

Additionally dosing nitrate directly (e.g. sodium nitrate) instead of dosing ammonia will also slightly increase alkalinity. 50ppm nitrate correspond to approx. 2.3dKH according to the linked article, so 11ppm nitrate would be enough to increase alkalinity by 0.5dKH once assimilated (this effect is delayed because it relies on the nitrate being assimilated by bacteria, algae, corals, etc.).

It's probably a whole set of different factors and I'm sure the suggested options are not enough to explain what is happening. Randy knows this stuff far better than I do, so if he suspects some third causative agent or simply coincidence, he is probably right.

Food for thought