There are quite a few things ways to set this up and even more strong opinions out there believing their approach is the only correct approach. Such is the aquarium hobby. I’ll share what works for me but I’m not the strong opinionated type unless it comes to cheep beer.
Here are a few of my opinions:
1. Don’t try to run the feed for the CaRx off of a manifold. There are too many variables with a manifold and with a CaRx more than most other aquarium equipment, stability is key. Either get some sort of dedicated feed pump like a maxijet, magdrive, or other pump as the feed and install a needle valve on the output (effluent drip) from the second stage into your sump. Control the driprate out. Or, a reliable peristaltic pump to feed water through. I use a medical grade peristaltic I bought on eBay. More expensive but rock solid and my preferred method. I just set/monitor mL per min. The Apex DOS might be a good solution but I’m not sure if it’s rated for 100% duty cycle?
2. Setting the drip rate / feed rate: I don’t know any more scientific way to approach this than to create a chart with all of the appropriate measurements. Ca, Mg, Alk, pH, bubble rate, CO2 pressure, salinity, date, time of day, etc... When I first fired up my CaRx, I stopped any two-part dosing because that can screw up measurements of how much I was contributing via the reactor. I took measurements of everything I could think of and then started at an arbitrary low feed rate on my peristaltic pump. I waited a day monitoring tank pH and then measured everything again. It took me over a month before I got to everything dialed in. Now I can go months without checking ANYTHING. Ca, Alk, Mg, pH are all spot on. You just have to interpret the numbers until you get to a level where Ca and Alk are where they should be without external intervention. Adjust as needed over time due to increased reef uptake.
I keep my media reactor at a stable pH through a Milwaukee pH monitor/controller. It opens the CO2 solenoid when the pH gets to high allowing CO2 into the CaRx dropping the pH. The same can be done with the Apex by allowing it to control the solenoid. I’m not an Apex code geek. As much Apex gear as I have on my system, I’m not overly impressed with their pseudo-code BS. So, I can’t be much help with making the Apex code extra special. In general, if pH is too high in the chamber, open solenoid. If not, close solenoid. Set the hysteresis.
The key is to make sure the reactor is low enough in pH to dissolve media but not so low that it turns it to mush. I use Reborn media with good success at 6.5. Once you reach that point, you just need to dial in the feed rate which is trial and error in my experience. If the CO2, regulator, and solenoid are set up to maintain that level reliably, the only thing left to adjust is throughput. What’s the coral uptake and how does that change over time? I mainly watch Alk. I’m not using the Ca reactor quite as much for Ca. It pretty much always stays at 420. I am adjusting the feed rate to maintain Alk and in turn, pH.
Disclaimer: there are other people that advocate for doing away with the controller portion altogether. They have luck with a much more low tech solution I won’t begin to explain. I don’t doubt that it works for them. I just have a different approach. If you’d like to read up on that solution, I can provide a link. However, it’s predicated on a very reliable peristaltic pump to feed the system which will run many hundreds of dollars alone. I’m somewhere in the middle.
Hope this helps. I’m sure there are other successful approaches. This is what works for me.