Jim_PA do you just have one return going back into your tank from your sump? Mine is like that and I have a 900gph pump and for some reason can't get the co2 to dissolve in the water at all. I have a 20" Rex Griggs reactor and not sure if its to much water pressure.
I'm asking you cause you seem to be the only other one in here that has a planted tank with a sump.
I have dual overflow on my tank, so I have 2 returns, one in each corner.
As for my plumbing; from the return pump in the sump, it goes to my reactor, from the reactor it goes into a Y, that then goes to each return. I am running MAG 18 (1800 GPH). I am using flex PVC for all my plumbing, also not using any 90's. All my plumbing is 1 inch. I don't have the MAG18 turned down at all so I am getting fair amount of turnover, also have my UV split from the MAG18, but that has a gatevalve to control flow. Assume you have PVC running from top to the bottom of your reactor? Inside of it?
As to help off gas of CO2, I have my sump covered with plexi glass, as for my overflows, I am running durso standpipe on each, and have some poret foam to help with the splash.
When you say it won't dissolve, do you mean your seeing the tiny & large bubbles in your tank, or that your drop checker won't turn green?
I had a problem with both at first, I filled my reactor up with a bunch of bio-balls real small ones, maybe half full with no water in it, and that helped with seeing large and most of the small bubbles in the tank.
If your drop checker won't turn green color, I had a lot of problems with that as well. It took a lot of adjustments to CO2, to get right balance. I cannot count the bubbles in my bubble counter. I have given up with that, and just adjust it to my drop checker is the right color.
Sorry this is so long, way more info than you wanted I am sure, but I figured I would give you full details of the setup.