Question on Alk, Ca, & Mg...no problem

[FLGator56]

...but something does not compute and want to be sure I am understanding this...

First - setup: I have a 39 CADLights / 12g refugium tank with Apex controlling 2 BRS dosing pumps, both confirmed at 1.1 ml / min. I use TM Bioactif salt, salinity is ~1.0265 by refractometer. Salifert test kits, Ca is 450 KH is 8.6 dKH, Mg is 1500. PH is currently 7.94 measured by probe (swing is 7.92 - 8.03). Tank has been up for almost 9 weeks. Light bioload but starting to stock. I use BRS recipe 1 for both.

Now my question - what is the relationship between Ca, Alk, and Ph in a fashion I can calculate? What I have inferred over the last 2 months is that as Ca goes down to 380 I can push dKH to 9.5 and get my PH to swing between 8.05 and 8.2 with no abiotic precip observed when dosing Alk. BUT, when I do a water change (5 gallons out of 37 gallon volume) the Ca, Alk, and PH swing back to the levels I first noted and when dosing Alk get Abiotic precip ( small amount that dissipates).

I had thought to get this precip...would have had to have higher dKH and higher PH...logically I would think I would have room to push Alk and thus get PH up while maintaining Ca. But in my tank (SouthFlorida closed house/ high CO2) the balance point seems to be right where I am at after my water change...

Long winded way of asking...how can represent this as an equation I can apply? I'm fine with my levels and do not want to chase PH, but if I can say...calculate that a CA of 430 let's me push dKH to 9.0 and Ph to 8.0-8.1 and get it stable then great. But I do not want to guess and want this stable...hence my request for someone that gets this to help me with the calculation. I've read some of Randy's articles but have only taken away the general understanding demonstrated above. Total noob on this topic. Thanks in advance!

 

[HighlandReefer]

Regarding pH in a reef tank, calcium levels do not play into the low pH problem. Alkalinity can play into the low pH problem as depicted in Randy's diagram:

Figure 1. The relationship between alkalinity and pH for seawater equilibrated with air containing normal and elevated carbon dioxide levels. The green dot shows natural seawater equilibrated with normal air, and the curves reflect the result that would be obtained if the alkalinity were artificially raised or lowered.


When you use Randy's recipe #1, baked baking soda, where this is dripped or added to the reef tank, there will be a local high pH in that area. It is this local high pH that can cause precipitation regardless of what the overall pH of the reef tank is (and calcium levels have little to do with this as long as they remain between the recommended guidelines).

Low pH: Causes and cures
http://reefkeeping.com/issues/2004-09/rhf/index.php

 

[bertoni]

We don't have a reasonable method of computing the carbon dioxide level in a tank, so we can't set up an equation to get the pH. As Cliff stated, the carbon dioxide level is the most important factor involved.

 

[FLGator56]

Thanks

Thanks

I realize CO2 plays a huge role, was hoping we could infer a relationship i.e describe through an equation. Very much appreciate the response.

 

[bertoni]

You're welcome!