took a completely different approach
I could not get CO2 to dissolve in this 200+ gallon system with a reef style overflow and sump. I tried most external reactors on the market, including the custom made one referenced above. These were 3 varieties of canister type reactors, where water and CO2 mix in a clear plastic container with or without an internal powerhead. All of these failed. None could move my drop checker (DC) from deep blue. The most expensive was the CarbonReactor EXT5000. Nor were Sera Flore or Ista units...all no help.
This week I replummed the entire system. I installed two Rex Grigg reactors (2" wide and 24" long) with bypasses on my two return lines from sump to display tank (DC). One CO2 tank and regulator to each. I reworked my drain lines and overflow box to have zero surface agitation. At first, no CO2. And then I turned up the low pressure adjuster on the regulators (btw CarbonDosers that work great), and then magic happened. In two hours DC yellow-green. I now have to worry about too much CO2. A good problem to have!