I use a calcium reactor( a kr2 I think ) Bought it used about a year and a half ago. I run it 24/7. I have not touched the settings since set up.
The calcium reactor alone even if properly set will still produce effluent in the 6.7 to 6.8 range. This can lower ph in the system and many who use them have persistent low ph in the 7.8 range. Corals can work in this if alkalinity is high. However, you could be flirting with 7.6 or 7.7. The low ph and CO2 that comes with it can spur cyano problems.etc.
I also dose limewater(kalk) for top off at night. It has a high ph of about 12.4 and will grab up CO2,as it forms calcium carbonate.
The 2 balance each other very well. My ph is steady at 8.22 low and 8.35 high point on any given day. Also by using the two sources, I don't have to push the calcium reactor very hard or tinker with it. I can always reduce the amount of limewater more easily if necessary in case the calcium and alkalinity are too high or use a little calcium chloride and carbonate aka2 part if it's too low. I have not had to take either of these steps in over a year.
People set the calcium reactors up in two different ways.
One is to use a controller to maintain the effluent ph, turning the flow of CO2 on and off with a controller.
I do it the other way.
First, set the effluent flow per manufacturer's recommendation for the unit. Do this before turning on the CO2.You attach a small pump/powerhead and adjust the amount of effluent by dialing a valve on the effluent output line(usually a piece or large airline tubing) to your sump. This step takes a measuring cup to catch the effleuent , a clock to time it and a little math to multiply it out instead of gathering effluent for and hour per setting to get litres or gph. Once you have it, let it run for a day and recheck. If it's good, you are ready to set the effluent ph.
You do this by turning on the CO2 supply and adjusting the amount of CO2 entering the water that will pass through the media by turning the needle valve attached to the CO2 regulator. You can get a general idea of CO2 volume by timing the bubbles(one bubble every e 2 or 3 or 4 sec,eg) in the bubble counter which is just a clear tube where the bubbles are visible. After a couple of hours catch some effluent in a cup and check the ph. When you settle it out at 6.7 or 6.8, you are set.Now let it run and leave it alone.
Recheck you effluent flow and effluent ph a few times over the next week and you should be all set.
A controller should be installed between the regulator and the input line for CO2 to the reactor. The controller is a ph monitor with a probe and a solenoid which will turn off and close off the CO2 supply when the ph drops too low in the system which is where I put the probe ansd not in the effluent.. In this way if there is excess CO2 for some unforseen reason the CO2 will be shut down when the ph in the aquarium reaches a preset low point (eg 8.0).
New media can be added when about half empty. Unless the media in your reactor is very muddy you can just put new media on top of the old.
A couple of other points. The effluent tube for the callcium reactor should be kept several inches out of the water to allow it to blow off excess CO2. It is also good to let it drip near the skimmer intake, since the skimmer will also oxygenate.
Good Luck
