OK some questions:
a) What do you refer to when you say drip an CO2 into the 10/40 area?
b) What brand or type of reactor is it?
c) What type of valve you are using to regulate the effluent flow? is it ball or needle valve?
d) Is the effluent valve at the inlet of the reactor or at the outlet?
e) If the regulatos does not have a bubble counter does the reactor has one?
f) Pics of the reactor and regulator will be very helpful specially the area of the regulator outlet, the effluent valve, the location of the reactor in relation to the sump and the recirculation pump and piping.
g) If you increase the CO2 bubble rate does it still slows down?
h) If you increase the effluent flow does it still slows down?
Some generic comments:
1. Usually maintaining a constant drip rate has to do with either the quality of the valve, maintaining a constant water pressure on the reactor inlet (The aqualifter may not have enough power to supply it), trying to keep too slow a drip or air being pumped into the reactor.
2. Maintaining a constant CO2 bubble rate usually has to do with low precision of the regulator, too high a discharge pressure with having to close the needle valve too much, faulty check valve or check valve that require too high of an opening pressure.
3. The JBJ regulator is a prefixed pressure one so you are right there is no way you can adjust the discharge pressure so you will have to live with 50 psi (lb/in^2) which IMO is rather high forcing you to close the needle valve too much.
4. The JBJ regulator usually has the check valve integrated into the bubble counter and the bubble counter and valve is removable as a set. I do not know of a version by JBJ without the bubble counter so in your case it is just probably missing. If so just insure you are really using a check valve. The system could work without a check valve but it is needed to insure that salt water can not back flow into the regulator damaging its components.
5. 6.7/6.8 PH is a good start point, do not get too concerned about the effluent dKh as youo will be adjusting PH and effluent flow to keep your tank alkalinity constant regardless of the effluent dKH. That measurment is made to detect basically only cases were the capacity of the regulator is being exceeded or to find out if something is really wrong but at this point you need to solve the issue of stability first.