kH is 8 and degassed pH is 7.5.
Even at 25cc/min you are losing a substantial amount of CO2 in the sump. From my experience with Cerges reactors, I found higher flow with back pressure is more efficient. I use the DCT 6000 as my main return pump. The Cerges is fed independently by a Ri0 1100. At 10%, your flow rate (assuming no head loss) is 158gph. The Rio is rated at 382 gph.
Suggestion: Drop the CO2 to about 20cc/min, up the DCT 6000 to 30-40% and partially close the valve on the reactor's exit. Play around with the flow rates first till you see no micro bubbles in the sump. Only then increase the CO2 to reach your desired pH drop in a shorter time.
If it's any consolation, setting up the Cerges and fine tuning it was the most frustrating thing for me during my initial set up. But be patient... work with one variable at a time and you will find the sweet spot!!
i was looking around last nite and i think the dct 6000 at the 10% speed setting is actually kinda of misleading. i also assumed 10% = 158 gph, man was i wrong lol
Found this chart on monsterfishkeepers:
and this from reef2reef:
DCT-6000 (42W) Maximum flow rate of up to 6,000 litres/hour - adjustable from 2,900 - 6,000 litres per hour in 10 steps.
•DCT-8000 (65W) Maximum flow rate of up to 8,000 litres/hour - adjustable from 2,500 - 8,000 litres per hour in 10 steps.
•DCT-12000 (85W) Maximum flow rate of up to 12,000 litres/hour - adjustable from 3,800 - 12,000 litres per hour in 10 steps.
2900 lph is about 766 gph.
used the headloss calculator from reefcentral (chose the rio 2500, similar gph to the dct 6000 on speed 1) and got 508 gph after head loss.
tbh i didn't expect fine tuning the co2 tbh such a pita with a huge reactor lol
i've also considered just grabbing a measuring cup filling it with water and inverting it in the tank or sump to measure the co2 through displacement, i think tom barr did this for his bigger tanks. think he aimed for 100 ml / min in his 120 or 180