Improperly set up calcium reactor?

Macdaddynick1 · Oct 9, 2017

After reading this article ( http://www.reefkeeping.com/issues/2004-09/rhf/ )

Specifically this part " In a sense, all that most aquarists need to know is that pH is a measure of the hydrogen ions in solution, and that the scale is logarithmic. That is, at pH 6 there is 10 times as much H+ as at pH 7, and that at pH 6 there is 100 times as much H+ as at pH 8. Consequently, a small change in pH can mean a big change in the concentration of H+ in the water. "

I am now wondering, whether running a calcium reactor at higher PH and higher flow instead of lower PH and lower flow, has lesser impact on the tank's overall pH?
For example, is running my reactor at (pH of 7.0 @ 60 ml/min) vs. (pH of 6.5 @ 20 ml/min) will reduce the overall pH drop from the use of the reactor?

thirst · Oct 9, 2017

I run my reactor at around 7

Randy Holmes-Farley · Oct 9, 2017

It's a very tricky mathematical issue and I'm not sure where the sweet spot is in terms of most efficient use of CO2 (that is, the most alk and calcium per unit of CO2), but I think it is probably at lower pH, not higher pH. The answer may even vary in ways that are not obvious (media particle surface area, etc.) since the chemistry may not be at any sort of equilibrium.

My reasoning is that it takes a certain amount of CO2 added to even get to the point where ANY calcium carbonate dissolves, and that is "wasted" CO2. The more water you need to bring to that point, the more waste CO2 there is. So a higher effluent rate may lead to less efficient use of CO2.

Once you have fixed the amount of water running through the system (the effluent rate) and the amount of CO2 added, then the higher you can get the pH (say, with a bigger media bed, recirculating passes through it, multiple media beds in series, etc.), the more efficient is the use of CO2.

Finally, I'd add that the pH effect on the aquarium is ONLY determined by the amount of CO2 added (when tank alk is kept stable), not by the pH of the effluent, which is also controlled by the amount of alkalinity dissolving into it). So higher pH effluent does not necessarily mean less impact on the tank pH.

Randy Holmes-Farley · Oct 9, 2017

Just to clarify that final point, it's not the H+ in the reactor effluent that is having the big impact on the tank pH. Pure water with a tiny bit of acid pushing it down to pH 6 will have almost no impact on tank pH. Like top off RO/DI that absorbs a bit of CO2 from the air and the pH drops to 6.

It is the net addition of CO2 over time that is the issue.

The reactor is adding HCO3- (and calcium) and the tank is consuming CO3-- (from calcification to form calcium carbonate). What is left over is acid.

CO2 + H2O --> H2CO3 (carbonic acid)

H2CO3 + CaCO3 --> 2HCO3- + Ca++ --> 2H+ + 2CO3-- + Ca++

then calcification:

Ca++ + CO3-- --> CaCO3

We are left with that 2H+ and CO3--, lowering pH.

Macdaddynick1 · Oct 10, 2017

Randy Holmes-Farley said:
Just to clarify that final point, it's not the H+ in the reactor effluent that is having the big impact on the tank pH. Pure water with a tiny bit of acid pushing it down to pH 6 will have almost no impact on tank pH. Like top off RO/DI that absorbs a bit of CO2 from the air and the pH drops to 6.

It is the net addition of CO2 over time that is the issue.

The reactor is adding HCO3- (and calcium) and the tank is consuming CO3-- (from calcification to form calcium carbonate). What is left over is acid.

CO2 + H2O --> H2CO3 (carbonic acid)

H2CO3 + CaCO3 --> 2HCO3- + Ca++ --> 2H+ + 2CO3-- + Ca++

then calcification:

Ca++ + CO3-- --> CaCO3

We are left with that 2H+ and CO3--, lowering pH.